Lemon Blueberry Sourdough Squares (Printable Copy)

Tangy moist squares with blueberries, lemon zest, sourdough starter, and crunchy streusel topping.

# What You 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 - 1.5 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:

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 between fingertips until mixture resembles 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 granulated sugar until light and fluffy, approximately 2-3 minutes. Beat in eggs one at a time until fully incorporated.
05 - Stir in sourdough starter, whole milk, lemon zest, lemon juice, and vanilla extract until combined. Mixture may appear slightly curdled.
06 - Add dry ingredients to wet ingredients and fold together until just combined, being careful not to overmix.
07 - Gently fold blueberries into batter using a rubber spatula.
08 - Spread batter evenly into prepared baking pan. Sprinkle streusel topping in an even layer across the surface.
09 - Bake for 35-40 minutes until a toothpick inserted in the center comes out clean and top is golden brown.
10 - Cool in pan for 15 minutes, then lift out using parchment paper overhang. Transfer to wire rack and cool completely before slicing into 12 squares.

# Expert Advice:

01 -
  • The sourdough starter adds a subtle sour note that makes the lemon sing without any bitterness.
  • These squares stay moist for days, making them perfect for grabbing on rushed mornings or wrapping up as gifts.
  • That cinnamon-brown sugar streusel creates a textural contrast that keeps you reaching for a second piece.
02 -
  • If you cut into warm cake, it will fall apart and you'll cry a little inside—wait until it's completely cool, even if it kills you.
  • Frozen blueberries are secretly better than fresh because they don't weep, and they thaw just enough during baking to be tender but still hold their shape.
03 -
  • Measure flour by spooning into the measuring cup and leveling with a knife—scooping directly from the bag compacts it and makes your cake dense.
  • Keep your sourdough starter out of the fridge for at least 30 minutes before using it so it's closer to room temperature and mixes more smoothly into the batter.
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