Lemon Blueberry Sourdough Squares (Print Version)

Tangy blueberry and lemon squares with 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 - 1.5 cups fresh or frozen blueberries, 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 It:

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 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 2-3 minutes. Beat in eggs one at a time, incorporating fully after each addition.
05 - Stir sourdough starter, milk, lemon zest, lemon juice, and vanilla into the creamed mixture until combined. The mixture may appear slightly curdled.
06 - Add dry ingredients to wet ingredients, mixing until just combined. Do not overmix.
07 - Gently fold blueberries into batter using a rubber spatula, preserving their shape.
08 - Spread batter evenly into prepared baking pan. Sprinkle streusel topping uniformly over the batter 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 pan for 15 minutes. Lift out using parchment paper overhang and cool completely on a wire rack. Cut into 12 squares before serving.

# Additional Tips::

01 -
  • The sourdough starter creates a surprisingly sophisticated tang that makes people ask what your secret ingredient is.
  • You finally have a legitimate reason to keep that starter alive and thriving on your counter.
  • It's easier to make than it looks, and one pan means minimal cleanup for maximum impressed guests.
02 -
  • Don't skip the 15-minute cooling period in the pan or your squares will crumble—this settling time is non-negotiable.
  • Room temperature ingredients blend together smoothly, while cold ingredients create lumps and an uneven crumb texture.
03 -
  • Keep your sourdough starter slightly neglected rather than perfectly fed—that unfed starter has the most character and creates the best tang.
  • Zest your lemon before cutting it open; the oils from fresh zest are worth more than any extract.
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