In a medium bowl, combine white sugar, brown sugar, flour, and cinnamon. Cube cold butter and mix everything together using your fingers or a pastry cutter until the mixture resembles coarse crumbs. Refrigerate the crumble while you prepare the batter.

Preheat your oven to 375°F (190°C). Grease a round or square baking pan with butter or non-stick spray.

In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt. Set this aside for later.

In another large bowl, beat the softened butter and granulated sugar until light and fluffy, about 3-5 minutes. Add the egg, mixing until just combined.

In a separate bowl, combine the sourdough starter, milk, lemon zest, and vanilla extract. Gradually add this mixture to the creamed butter and sugar, mixing on low speed just until combined.

Gradually add the dry ingredients to the wet mixture in two halves, gently mixing until just combined. Avoid over-mixing.

Toss your blueberries with a sprinkle of flour in a bowl. Gently fold the floured blueberries into the batter.

Pour half of the batter into the prepared baking pan. Sprinkle the blueberries on top and pour the remaining batter over them. Finish with the reserved crumble topping.

Transfer to the oven and bake for 45-50 minutes, or until golden brown and a toothpick comes out clean.

Allow to cool in the pan for about 10-15 minutes, then transfer to a wire rack to cool completely.