Lemon Blueberry Sourdough Squares (Printable)

Moist, tangy squares with blueberries, lemon zest, sourdough, and a 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

# How To Make:

01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, allowing excess to hang over edges 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, cream softened butter and sugar until light and fluffy, approximately 2 to 3 minutes. Beat in eggs one at a time, incorporating each fully before adding the next.
05 - Stir sourdough starter, milk, lemon zest, lemon juice, and vanilla into the butter mixture until combined. The batter may appear slightly curdled.
06 - Add dry ingredients to wet ingredients and mix until just combined, taking care not to overmix.
07 - Gently fold blueberries into the batter using a rubber spatula.
08 - Spread batter evenly into the prepared pan. Sprinkle streusel topping evenly across the surface.
09 - Bake for 35 to 40 minutes, or until a toothpick inserted into the center emerges clean and the top is golden brown.
10 - Cool in the pan for 15 minutes. Using parchment overhang, lift cake from pan and transfer to a wire rack. Cool completely before slicing into 12 squares.

# Top Tips:

01 -
  • The sourdough starter adds a subtle tang that keeps the sweetness from feeling one-note, making you want another square before the first has settled.
  • Blueberries stay juicy and burst softly rather than turning into little rocks, which honestly changes everything about the texture.
  • You probably have most of this in your kitchen right now, and the whole thing comes together in under an hour from idea to cooling rack.
02 -
  • If your blueberries are frozen, leave them frozen when you fold them in because thawed berries release juice and turn the batter purple in an unappealing way.
  • The sourdough starter can be discard or active starter at any stage of fermentation; the acidity is what mattersthe flavor doesn't vary much regardless, so use what you have.
  • Softening the butter to room temperature matters more than you'd think; cold butter won't cream properly and you'll get a denser, less tender crumb.
03 -
  • Measure flour by spooning it into your measuring cup and leveling with a knife rather than scooping directly from the bag, which compacts the flour and can make the cake dense.
  • If you don't have sourdough starter, you can substitute ½ cup plain Greek yogurt plus ½ cup milk for a similar tang and moisture, though the flavor will be slightly different.
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