Lemon Blueberry Sourdough Squares (Print Version)

Tangy lemon and blueberry squares with sourdough and a crunchy streusel topping, perfect for any time enjoyment.

# Ingredient List:

→ 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 1/2 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

# 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 forms pea-sized crumbs. Refrigerate until needed.
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.
05 - Beat in eggs one at a time, mixing well after each addition.
06 - Stir in sourdough starter, milk, lemon zest, lemon juice, and vanilla extract until combined. Mixture may appear slightly curdled.
07 - Add dry ingredients to wet ingredients and mix until just combined. Do not overmix.
08 - Gently fold blueberries into batter using a rubber spatula.
09 - Spread batter evenly into prepared baking pan. Sprinkle streusel topping evenly over batter surface.
10 - Bake for 35-40 minutes until a toothpick inserted in the center comes out clean and top is golden brown.
11 - Cool in pan for 15 minutes. Lift out using parchment overhang and cool completely on wire rack before slicing into 12 squares.

# Helpful Hints:

01 -
  • The sourdough starter adds a sophisticated tang that makes people ask for your secret ingredient without knowing it's hiding right there in plain sight.
  • You get a genuinely moist crumb that stays soft for days, meaning these squares are even better the next morning with your coffee.
  • The streusel topping gives you that satisfying crunch against the tender cake, plus it looks impressive without any fancy techniques.
02 -
  • Do not skip the 15-minute pan cooling step, even when the cake smells incredible and you're desperate to dig in—this is when the structure sets enough to hold together when you slice it.
  • Your sourdough starter doesn't need to be freshly fed or at peak activity; in fact, using discard or unfed starter is often preferable because the slightly less vigorous starter still adds tang without causing the cake to rise too aggressively and then collapse.
  • If your batter looks curdled after adding the sourdough and milk, this is not a problem and not a sign anything went wrong—the acid in the sourdough naturally interacts with the milk, and mixing in the flour will bring everything back together smoothly.
03 -
  • Use an offset spatula or the back of a spoon dipped in water to smooth the batter into an even layer before topping—this ensures even baking and makes the finished cake look more polished.
  • If you're doubtful about your sourdough starter, remember that even weak or unfed starter adds flavor and contributes to the chemistry of the cake; there's no version of this recipe where it won't work beautifully.
  • Zest your lemon over the dry ingredients first so any oils stay in the flour, then juice it separately into a small bowl before adding it to the wet ingredients—this prevents you from accidentally mixing citrus pulp into everything.
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