Lemon Blueberry Sourdough Squares (Printable)

Tangy lemon and juicy blueberries meet sourdough in moist, tender squares with a crunchy topping.

# Ingredient List:

→ Cake Batter

01 - ½ cup unsalted butter, softened
02 - ¾ cup granulated sugar
03 - 2 large eggs
04 - ½ cup sourdough starter, unfed or discard
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 - ½ teaspoon baking soda
12 - ½ teaspoon fine sea salt
13 - 1½ cups fresh blueberries or frozen, unthawed

→ Streusel Topping

14 - ¼ cup unsalted butter, cold and diced
15 - ⅓ cup light brown sugar, packed
16 - ½ cup all-purpose flour
17 - ½ teaspoon ground cinnamon
18 - Pinch of salt

# Steps:

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 together with 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.
04 - In a large bowl, cream softened butter and sugar together until light and fluffy, approximately 2-3 minutes. Beat in eggs one at a time, incorporating each fully.
05 - Stir in sourdough starter, milk, lemon zest, lemon juice, and vanilla extract until combined. The mixture may appear slightly curdled at this stage.
06 - Add dry ingredients to wet ingredients and mix until just combined, avoiding overmixing.
07 - Gently fold blueberries into the batter using a spatula.
08 - Spread batter evenly into prepared pan. Sprinkle streusel topping evenly over batter surface.
09 - Bake for 35-40 minutes until a toothpick inserted into center comes out clean and top is golden brown.
10 - Cool in pan for 15 minutes, then lift out using parchment paper and transfer to wire rack. Cool completely before slicing into 12 squares.

# Expert Advice:

01 -
  • The sourdough starter adds a gentle tang that makes the blueberries sing without needing extra vanilla or spice.
  • These squares stay moist for days, and the crunchy streusel keeps its texture even after sitting in a container.
  • It's forgiving enough for a weekday bake but impressive enough to bring to someone's house and feel proud about it.
02 -
  • Don't skip the cooling step in the pan; those first 15 minutes are when the cake firms up enough to handle the weight of the streusel without falling apart.
  • Sourdough starter behaves differently depending on when you feed it, so the batter might be slightly thicker or thinner than you expect—if it seems too thick, add a tablespoon of milk; too thin, add a tablespoon of flour.
03 -
  • Toast the streusel ingredients in a dry pan for 2 minutes before combining if you want extra nuttiness and deeper caramelization during baking.
  • Brush the warm cake lightly with lemon simple syrup (equal parts lemon juice and sugar, warmed together) right after it comes out of the oven for an extra glossy finish and subtle tang.
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