Lemon Blueberry Sourdough Squares (Printable)

Moist squares featuring blueberries, lemon zest, sourdough depth, and crunchy streusel topping.

# 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 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

# 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 sugar until light and fluffy, approximately 2 to 3 minutes. Beat in eggs one at a time, ensuring full incorporation.
05 - Stir in sourdough starter, whole milk, lemon zest, lemon juice, and vanilla extract until combined. The mixture may appear slightly curdled at this stage.
06 - Add dry ingredients to wet ingredients and mix until just combined. Do not overmix.
07 - Gently fold blueberries into the batter using a spatula, being careful not to break the berries.
08 - Spread batter evenly into the prepared pan. Sprinkle streusel topping evenly over the batter surface.
09 - Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown.
10 - Cool in the pan for 15 minutes, then lift out using parchment overhang and cool completely on a wire rack. Cut into 12 squares before serving.

# Tips from the Pros:

01 -
  • The sourdough starter adds a subtle tang that makes people ask what your secret is, and you get to smile mysteriously while saying nothing.
  • One pan, minimal cleanup, and the streusel topping means you get that satisfying crunch with every bite without extra steps.
  • It comes together in under an hour total, so you can serve warm squares with coffee before anyone realizes you just made something actually impressive.
02 -
  • Don't thaw frozen blueberries before adding them; they release moisture when thawed and will sink and stain the batter instead of staying distributed throughout.
  • The batter will look slightly curdled after you add the sourdough starter and milk, and this is completely normal; the acid in the sourdough reacting with the other ingredients is actually what you want for tenderness.
  • Measuring flour by spooning and leveling rather than scooping directly matters more than you'd think; one scoop can add 15-20 grams more flour than needed, which makes the cake dense and dry.
03 -
  • Cut your squares while the cake is still slightly warm so the edges stay clean and defined; cold cake tends to crumble at the corners.
  • Store covered at room temperature for up to three days, and these actually taste better on day two when the flavors have settled and deepened slightly.
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