Step 1

Prepare the crumb mixture by adding melted and cooled butter, brown sugar, cinnamon, and flour to a medium bowl. Mix until combined, then set to the side.

Step 2

In the bowl of a stand mixer with a paddle attachment, cream the room-temperature butter, sugar, and brown sugar.

Step 3

Add the sour cream, eggs, vanilla, sourdough starter, and milk, blending until smooth.

Step 4

In a separate bowl, combine salt, baking soda, baking powder, and flour. Add these dry ingredients to the wet ingredients, then mix on low speed until just combined. You don't want to overmix the batter.

Step 5

Preheat the oven to 350 degrees and lightly grease a 9x13 baking dish. Toss the blueberries with two teaspoons of flour to coat, a step that will help them not to sink to the bottom.

Step 6

Pour half of the cake batter into the baking dish, sprinkle half of the blueberries on top, then follow with half of the crumb mixture.

Step 7

Gently fold the rest of the blueberries into the remaining batter, then pour over the crumble in the baking dish. Sprinkle the rest of the crumb topping over the top of the batter.

Step 8

Bake in the preheated oven for 40-50 minutes or until the edges turn golden brown, the center is set, and a toothpick inserted in the middle of the cake comes out clean. Cool on a wire rack before serving.