Step 1

Preheat the oven to 350 F/176 C. Lightly oil an 8×8-inch pan and line with parchment paper. Smooth the bottom and sides of the pan so the paper is flat.

Step 2

Add the blueberries to a small bowl. Sift a a tablespoon of powdered sugar over the top and gently toss to combine. This will prevent the berries from sinking into the cake. Set aside.

Step 3

To make the crumble topping, add all of the ingredients to a medium-sized bowl. Mix into crumbles using your hands. The butter should be well blended into to the flour; some larger pieces are OK. If using frozen crumble topping, remove it from the freezer now.

Step 4

In a small skillet, melt the butter until light golden brown around the edges (you are not fully browning the butter). This will only take a few minutes, but do not walk away from the pan- it will burn quickly! Set aside to cool slightly.

Step 5

Meanwhile, using a stand mixer, beat the sugar, egg, and sourdough starter on medium-low speed, about 1 minute. With the machine running, gradually pour in the warm melted butter.

Step 6

In a separate bowl, sift the dry ingredients together. Add to the wet ingredients, and mix on low speed until just combined. You should still see specks of flour in the bowl; do not over mix. Add the sour cream and mix until smooth. This technique will give the cake a soft, velvety texture. The batter will be thick.

Step 7

Spoon the batter into the pan. In an even layer: scatter 1/2 of the crumbles over the top first, then layer 1/2 of the blueberries on top. Repeat to finish all of the crumbles and blueberries.

Step 8

Bake for 55-65 minutes or until a toothpick comes out clean when inserted (I use a dried piece of spaghetti). For me, 60 minutes is the sweet in a shiny metal pan. Please check at the 55 minute mark to see how things are going.

Step 9

Cool completely before serving.

Step 10

Dust with powdered sugar if you like, and cut into squares.