A tender, buttery coffee cake enriched with sourdough starter, bursting with juicy blueberries and finished with a brown sugar–cinnamon streusel. Soft crumb, bright berries, and a gentle tang make it perfect for breakfast or dessert.
Preheat the oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish.
Make the crumb topping: In a medium bowl, combine the melted and cooled butter, brown sugar, cinnamon, and flour; mix until combined and set aside.
In the bowl of a stand mixer fitted with the paddle, cream the softened butter, granulated sugar, and brown sugar.
Add the sour cream, eggs, vanilla extract, sourdough starter, and milk; blend until smooth.
In a separate bowl, whisk together the salt, baking soda, baking powder, and flour.
Add the dry ingredients to the wet and mix on low just until combined; do not overmix.
Toss the blueberries with 2 teaspoons of flour to coat.
Spread half of the batter into the prepared pan; sprinkle with half of the blueberries, then half of the crumb topping.
Gently fold the remaining blueberries into the remaining batter; spread over the crumb layer. Sprinkle the remaining crumb topping evenly over the batter.
Bake for 40–50 minutes, until the edges are golden, the center is set, and a toothpick inserted in the center comes out clean.
Cool on a wire rack before slicing and serving.