Lemon Blueberry Sourdough Squares (Print)

Moist tangy squares with juicy blueberries, lemon zest, and sourdough depth under a crunchy streusel topping.

# Components:

→ 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

# Directions:

01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving overhang for easy removal.
02 - In a small bowl, combine flour, brown sugar, cinnamon, and salt. Add cold butter and rub with fingertips until mixture forms pea-sized crumbs. Refrigerate until needed.
03 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
04 - In a large bowl, cream butter and sugar until light and fluffy, approximately 2-3 minutes. Beat in eggs one at a time.
05 - Stir in sourdough starter, milk, lemon zest, lemon juice, and vanilla until combined. Mixture may appear slightly curdled.
06 - Add dry ingredients to wet ingredients and mix until just combined.
07 - Gently fold blueberries into batter.
08 - Spread batter evenly into prepared pan. Sprinkle streusel topping evenly over batter.
09 - Bake for 35-40 minutes or 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 and cool completely on wire rack before slicing into squares.

# Expert Advice:

01 -
  • The sourdough starter adds a whisper of tanginess that balances the sweetness, making these squares feel sophisticated without tasting sour.
  • Blueberries stay plump and juicy while the lemon zest keeps everything bright, so you won't feel weighed down after eating a square.
  • That streusel topping gives you a satisfying crunch that makes the whole experience feel more indulgent than it actually is.
02 -
  • Room-temperature ingredients mix together far more smoothly than cold ones; pull your eggs and butter out 30 minutes before starting and you'll get a better-emulsified batter with fewer dense pockets.
  • Sourdough starter that's been sitting in your fridge unfed is actually perfect for this—you don't want the extra gas from an actively bubbling starter, which can make the crumb too open and fragile.
  • The lemon juice is doing double duty: flavoring the cake and providing acid that reacts with the baking soda for extra rise, so don't reduce it thinking it will be too sour.
03 -
  • Toast pecans or almonds and add them to the streusel for a nutty complexity that pairs beautifully with lemon and sourdough, adding about ½ cup to the mix.
  • Use a microplane zester for lemon zest so you get the bright oils without the bitter white pith, and measure by gently pressing it down rather than aggressively packing it.
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