Lemon Blueberry Sourdough Squares (Printable)

Tangy lemon and blueberry squares with sourdough depth and a crunchy streusel topping, ideal for any time indulgence.

# 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, discard or active 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 or frozen 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, leaving overhang on sides for easy removal.
02 - In a small bowl, combine flour, brown sugar, cinnamon, and salt. Add cold diced butter and rub mixture between fingertips until pea-sized crumbs form. 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, beat softened butter and granulated sugar together for 2 to 3 minutes until light and fluffy. Add eggs one at a time, beating well after each addition.
05 - Stir in sourdough starter, milk, lemon zest, lemon juice, and vanilla extract until combined. Mixture may appear slightly curdled.
06 - Add dry ingredient mixture to wet ingredients and mix until just combined. Do not overmix.
07 - Gently fold blueberries into batter using a rubber spatula.
08 - Spread batter evenly into prepared baking pan. Sprinkle chilled streusel topping evenly across batter surface.
09 - Bake for 35 to 40 minutes until a toothpick inserted in the center comes out clean and the top is golden brown.
10 - Cool in pan for 15 minutes. Lift out using parchment paper overhang and transfer to wire rack to cool completely before cutting into 12 squares.

# Expert Advice:

01 -
  • The sourdough starter adds this subtle tang that balances the bright lemon without making anything taste sour.
  • You can actually use unfed starter from the discard jar, so nothing goes to waste.
  • Streusel topping gets wonderfully crunchy while the cake stays moist underneath, which is genuinely hard to pull off.
02 -
  • Don't thaw frozen blueberries before folding them in—cold berries stay firmer and won't bleed as much color into the batter.
  • The sourdough starter doesn't need to be active or freshly fed; that discard you're about to throw away is exactly what you want here.
  • If your cake seems to brown too quickly on top, tent it loosely with foil for the last 10 minutes of baking.
03 -
  • If you don't have a 9-inch square pan, you can use a 9-by-13-inch rectangular pan and adjust the thickness and baking time accordingly.
  • Room-temperature eggs and milk incorporate more smoothly into the batter than cold ones, so pull them out 30 minutes before baking.
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