Lemon Blueberry Sourdough Squares

My sourdough starter was acting particularly lively one Sunday morning, and I found myself standing in the kitchen wondering what to do with the excess I'd just discarded. That's when it hit me—why not bake it into something that would actually celebrate all that tangy, complex flavor instead of letting it go to waste? I'd been craving something between a coffee cake and a pound cake, something with real substance and brightness. The lemon and blueberries came together naturally, like they'd been waiting for this exact moment. That first batch turned out exactly as I'd imagined: moist, tender, with a subtle sourness that made the citrus sing even louder.

I made these for my neighbor Sarah when she was recovering from surgery, and watching her face light up when she bit into one made the whole morning of baking feel worthwhile. She called me three days later to say she'd finished the entire pan and wanted to know if I could teach her the recipe. There's something about a cake that's this easy to make but tastes like you spent hours perfecting it.

Ingredients

• Unsalted butter (120 g): Room temperature is crucial here, so pull it out of the fridge while you're gathering everything else and let it soften naturally—it creams much better than cold butter and gives you a lighter, more tender crumb.
• Granulated sugar (150 g): The sweetness needs to be straightforward so the lemon and sourdough tang can stand out; this amount hits that balance perfectly.
• Eggs (2 large): They should be at room temperature too, which I learned the hard way when cold eggs made my batter look oddly separated and greasy until it came together in the oven anyway.
• Sourdough starter (120 g): Use whatever you have, whether it's the discard from your regular feeding or active starter—the magic is in that tang, and either version delivers it beautifully.
• Whole milk (240 ml): This keeps the crumb tender and plays nicely with the sourdough's acidity, creating that perfect balance between tangy and mellow.
• Lemon zest (from 1 lemon): Don't skip this—it's where the brightness lives, so zest generously and use it in the batter where it can infuse every bite.
• Fresh lemon juice (2 tbsp): Sharp and vital, this cuts through the richness and makes the whole cake feel lighter than it actually is.
• Vanilla extract (1 tsp): Just enough to round out the flavors without making anyone guess what it is.
• All-purpose flour (250 g): Measure by weight if you can; it's the only way to guarantee consistent results and the difference is genuinely noticeable.
• Baking powder (2 tsp) and baking soda (½ tsp): The baking powder does the main lifting, but the baking soda reacts with the sourdough's acidity to give you extra rise and a subtly different flavor development.
• Fine sea salt (½ tsp): This enhances everything, making the lemon brighter and the blueberries taste more like themselves.
• Fresh blueberries (200 g): Use frozen if you're out of season and don't thaw them—the ice crystals actually help them stay whole and not bleed color everywhere, which I discovered by accident.
• Cold unsalted butter for streusel (50 g): Keep it cold and diced small so it stays distinct in the topping and creates those gorgeous crunchy bits.
• Light brown sugar for streusel (65 g): The molasses gives the topping a subtle depth that plain sugar never could.
• All-purpose flour for streusel (65 g): Weight matters here too, since too much makes the topping hard and cakey instead of delightfully crumbly.
• Ground cinnamon (½ tsp): A whisper of warmth that complements the lemon without competing with it.

Instructions

Set your oven and pan:

Get your oven heating to 180°C (350°F) and line a 23cm square baking pan with parchment paper, leaving enough overhang on two sides that you can grab it later and lift the whole cake right out. This step saves you from the frustration of trying to pry hot cake from the corners.

Make the streusel first:

In a small bowl, whisk together the flour, brown sugar, cinnamon, and salt, then add your cold diced butter and rub it all together with your fingertips until it looks like scattered peas. Pop it in the fridge while you work on the batter—the cold helps it stay crunchy and separate in the oven instead of melting into a solid mass.

Combine your dry ingredients:

In a medium bowl, whisk the flour, baking powder, baking soda, and salt together, breaking up any small lumps you find hiding in the baking soda. Set this aside and try not to overthink it—this is the simplest part of the whole process.

Cream the butter and sugar:

In your large mixing bowl, beat the softened butter and sugar together for about 2 to 3 minutes until it's noticeably lighter in color and fluffy, which is when you know the air has been properly incorporated. Then beat in each egg one at a time, letting each one fully blend before adding the next.

Bring in the sourdough and wet ingredients:

Stir in your sourdough starter, milk, lemon zest, lemon juice, and vanilla until everything is combined—the mixture might look a bit curdled or separated, which is completely normal and will even out once the flour hits it. Don't panic at this stage because it always looks slightly off, but it's actually exactly right.

Mix wet and dry gently:

Add the dry ingredients to the wet ingredients and mix until just combined, which means you should see no streaks of flour but shouldn't overmix to the point where everything becomes tough. This is where restraint actually matters because overmixing is the enemy of tender cakes.

Fold in the blueberries:

Gently fold the blueberries into the batter, being careful not to crush them or stir too much—they'll bleed color if you're rough with them, which actually tastes fine but looks less beautiful. If you're using frozen blueberries, don't thaw them first because the ice helps them stay whole.

Spread and top:

Pour the batter into your prepared pan and smooth it into an even layer, then sprinkle the chilled streusel topping evenly over the entire surface. If the streusel has warmed up too much while you were mixing the batter, pop it back in the fridge for a minute to firm up again.

Bake until golden:

Slide the pan into your preheated oven and bake for 35 to 40 minutes, checking at 35 minutes by inserting a toothpick into the center—it should come out clean or with just a few moist crumbs clinging to it, and the top should be golden brown. Every oven is different, so use your eyes and the toothpick test rather than just watching the clock.

Cool with patience:

Let the cake rest in the pan for about 15 minutes, then use the parchment overhang to lift it straight out onto a wire rack where it can cool completely before you slice it. Cutting into it while it's still warm will make the squares fall apart, but waiting for it to cool fully means clean, beautiful cuts.

Save

My mother tasted these after my sister brought them to a family dinner, and she immediately asked if I could make them for her book club meeting the following week. Seeing my own creation become something my family asked for, requested even, shifted how I thought about baking—it wasn't just about the process or the flavors anymore, it was about creating something that meant something to people I loved.

When to Bake These Squares

These are spectacular on a lazy Saturday morning when you have time to sit with a slice and actually taste it, but they're equally suited to a weeknight bake when you want something ready-to-grab for the next few days. I've found that making these on Sunday afternoon means Monday through Wednesday mornings are instantly better because there's cake waiting in the kitchen. They're sturdy enough to pack in a lunchbox or bring to a friend's house, yet delicate enough to feel like a proper treat rather than just another snack.

Storing Your Cake Squares

These squares actually improve over a day or two as the sourdough tang becomes more pronounced and the cake settles into an even more tender structure. Keep them in an airtight container at room temperature for up to four days, or wrap them individually and freeze them for up to three weeks—just thaw at room temperature for an hour and they taste virtually identical to freshly baked. I like to make these when I know I'll want cake later in the week because the anticipation of having it waiting in the kitchen never gets old.

Make It Your Own

This cake is actually quite flexible if you want to play with variations, and I've learned that small changes can yield surprisingly different results. The lemon and blueberry combination is wonderful as written, but I've made delicious versions with lime juice instead of lemon, or with raspberries mixed in alongside the blueberries for extra tartness. You can also toast and chop some pecans and fold them into the streusel for a nuttier crunch, though the original is already perfectly textured.

• For a nuttier streusel, toast ½ cup chopped pecans and fold them into the topping before it goes on the batter.
• Swap the lemon for lime juice and zest if you want a more tropical bright note, or use a combination of both citrus fruits.
• Fresh or frozen raspberries can replace up to half the blueberries if you want extra tartness and a more delicate structure.

Save

These squares have become my answer to the question of what to bake when I want something that feels special without being complicated, and they've taught me that the best recipes are the ones you actually want to make again and again. They're waiting for you to discover your own kitchen moment with them.

Recipe FAQ

→ What role does sourdough starter play in this dish?

The sourdough starter adds a subtle tang and depth, enhancing the flavor and texture while keeping the squares moist and tender.

→ Can I use frozen blueberries instead of fresh?

Yes, frozen blueberries can be folded in directly without thawing to prevent the batter from becoming too wet.

→ How does the streusel topping affect the final squares?

The streusel provides a crunchy, cinnamon-flavored contrast to the soft and moist interior, adding texture and sweetness.

→ What’s the best way to store these squares?

Store in an airtight container at room temperature for up to two days or refrigerate for a longer shelf life.

→ Can I add nuts to this preparation?

Yes, adding chopped toasted pecans to the streusel enhances flavor and adds a nutty crunch.