Lemon Blueberry Coffee Squares (Printable)

Bright lemon and fresh blueberries combine with sourdough and a crunchy streusel for a moist, flavorful treat.

# 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

# Directions:

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 between fingertips until mixture forms pea-sized crumbs. Transfer to refrigerator 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 granulated sugar until light and fluffy, approximately 2-3 minutes using an electric mixer or vigorous whisking.
05 - Add eggs one at a time to the butter mixture, beating well after each addition until fully incorporated.
06 - Stir in sourdough starter, milk, lemon zest, lemon juice, and vanilla extract until combined. The mixture may appear slightly curdled, which is normal.
07 - Add dry ingredient mixture to wet ingredients, folding gently with a rubber spatula until just combined. Do not overmix.
08 - Gently fold blueberries into batter using minimal strokes to avoid breaking the fruit.
09 - Spread batter evenly into prepared pan. Sprinkle refrigerated streusel topping evenly across the surface.
10 - Bake for 35-40 minutes until a toothpick inserted in the center emerges clean and the top is golden brown.
11 - Allow cake to cool in pan for 15 minutes. Lift out using parchment overhang and transfer to wire rack to cool completely before slicing into 12 squares.

# Expert Tips:

01 -
  • The sourdough starter adds a subtle tang and moisture that regular butter cakes never quite achieve, making you wonder why you haven't been doing this all along.
  • Juicy blueberries stay plump and don't sink mysteriously to the bottom like they have their own agenda.
  • That crunchy streusel topping creates the most satisfying textural contrast, and honestly, people will think you're more skilled than you actually are.
02 -
  • That slightly curdled look when you combine the sourdough and milk into the creamed butter is not a mistake—the acidity actually breaks it apart slightly, and it comes back together perfectly fine once you add the flour.
  • Frozen blueberries should never be thawed before folding in, because the ice crystals inside them keep them from bleeding all their color and juice into the batter and turning everything gray.
03 -
  • Measure your flour by weight (grams) if you have a scale, because scooping and leveling by volume can vary wildly and affect your cake's texture.
  • Don't thaw frozen blueberries, and consider tossing them in a tablespoon of your measured flour before folding them in—this keeps them from sinking to the bottom during baking.
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