Lemon Blueberry Sourdough Squares (Printable)

Tangy lemon and juicy blueberry squares with sourdough and a crunchy streusel topping.

# What You'll Need:

→ Cake Batter

01 - 1/2 cup unsalted butter, softened
02 - 3/4 cup granulated sugar
03 - 2 large eggs
04 - 1/2 cup sourdough starter, unfed
05 - 1 cup whole milk
06 - Zest of 1 lemon
07 - 2 tablespoons fresh lemon juice
08 - 1 teaspoon vanilla extract
09 - 2 cups all-purpose flour
10 - 2 teaspoons baking powder
11 - 1/2 teaspoon baking soda
12 - 1/2 teaspoon fine sea salt
13 - 1 1/2 cups fresh blueberries

→ Streusel Topping

14 - 1/4 cup unsalted butter, cold and diced
15 - 1/3 cup light brown sugar, packed
16 - 1/2 cup all-purpose flour
17 - 1/2 teaspoon ground cinnamon
18 - Pinch of salt

# How to Make It:

01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, allowing excess to hang over edges for easy removal.
02 - In a small bowl, combine flour, brown sugar, cinnamon, and salt. Add cold diced butter and rub between fingertips until mixture forms pea-sized crumbs. Refrigerate until ready to use.
03 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
04 - In a large bowl, cream softened butter and sugar until light and fluffy, approximately 2-3 minutes. Beat in eggs one at a time until fully incorporated.
05 - Stir in sourdough starter, milk, lemon zest, lemon juice, and vanilla until combined. The mixture may appear slightly curdled.
06 - Add dry ingredient mixture to wet ingredients and mix until just combined, being careful not to overmix.
07 - Gently fold blueberries into the batter using a rubber spatula.
08 - Pour batter evenly into prepared baking pan. Sprinkle streusel topping uniformly over the batter.
09 - Bake for 35-40 minutes until a toothpick inserted in the center comes out clean and the top is golden brown.
10 - Cool in the pan for 15 minutes. Lift out using parchment paper and cool completely on a wire rack before slicing into squares.

# Expert Advice:

01 -
  • The sourdough starter adds a subtle tang that makes the lemon sing without tasting sour or one-dimensional.
  • These squares stay moist for days, which means you can actually enjoy them beyond that first bite.
  • Frozen blueberries work just as well as fresh, so you're not locked into a season or a trip to the market.
02 -
  • Don't use your sourdough starter straight from active feeding—it can overpower the lemon—so discard or unfed starter is actually your friend here.
  • The secret to a moist square that lasts days is the sourdough starter and milk combination, which creates a tender crumb that doesn't dry out as quickly as regular coffee cake.
  • Frozen blueberries are genuinely better than thawed ones because they don't bleed color into the batter, keeping everything looking bright and intentional.
03 -
  • Room temperature ingredients mix together smoothly and create better texture, so pull eggs, butter, and milk out of the fridge 30 minutes before you start.
  • If your streusel topping isn't staying cold while you're prepping the batter, stick the bowl back in the fridge for 5 minutes—cold butter is what creates those crunchy, distinct crumbs instead of a dense, oily layer.
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