Lemon Blueberry Sourdough Squares (Printer View)

Tangy lemon and juicy blueberry squares with subtle sourdough and 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, active 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 - 1/2 teaspoon baking soda
12 - 1/2 teaspoon fine sea salt
13 - 1 1/2 cups fresh blueberries, or frozen unthawed

→ 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

# Method:

01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, allowing 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 sugar until light and fluffy, approximately 2-3 minutes. Beat in eggs one at a time, incorporating fully after each addition.
05 - Stir sourdough starter, milk, lemon zest, lemon juice, and vanilla extract into the butter mixture until combined. The batter may appear slightly curdled, which is normal.
06 - Fold dry ingredients into wet mixture until just combined, avoiding overmixing.
07 - Gently fold blueberries into the batter using a rubber spatula.
08 - Spread batter evenly into the prepared pan. Sprinkle chilled streusel topping uniformly over the surface.
09 - Bake for 35-40 minutes until a toothpick inserted into the center emerges clean and the top is golden brown.
10 - Cool in the pan for 15 minutes. Lift out using parchment paper overhang and transfer to a wire rack. Cool completely before cutting into 12 squares.

# Expert Advice:

01 -
  • The sourdough starter gives you that subtle tang without any extra fermentation time, which feels like a delicious shortcut.
  • Those blueberries burst into little flavor pockets while staying whole, and the crunchy streusel top is honestly addictive.
  • It's forgiving enough for a lazy Sunday baking session but impressive enough to share with someone whose opinion actually matters to you.
02 -
  • Don't thaw frozen blueberries—I learned this the hard way when they bled their color throughout the batter and tasted mushy, whereas frozen ones stay whole and burst in little pockets.
  • The sourdough starter needs to be unfed or your discard, not at peak ripeness, because you're not trying to ferment the cake, just add that subtle tangy depth.
  • Room temperature eggs and softened butter actually matter here because cold ingredients won't cream properly and you'll end up with a denser, less fluffy cake.
03 -
  • Make the streusel topping first and refrigerate it while you prep everything else—this keeps it cold and ensures maximum crunchiness in the final bake.
  • Don't open the oven door to peek during baking; wait until at least minute 30, because temperature fluctuations can prevent proper rising and browning.
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