Combine white sugar, brown sugar, flour, and cinnamon. Add cubed cold butter and work until it resembles coarse crumbs. Chill while preparing the cake batter.

Preheat oven to 375°F (190°C). Grease a baking pan and optionally dust with flour.

Whisk together all-purpose flour, baking powder, and salt until fully combined.

Beat the softened butter and granulated sugar in a stand mixer until light and fluffy. Add the egg and mix until just combined.

Mix together sourdough starter, milk, lemon zest, and vanilla extract until well combined.

Gradually add the wet mixture to the butter mixture, alternating with dry ingredients, mixing on low speed until just combined.

Toss the blueberries lightly with flour to prevent sinking, then gently fold into the batter.

Spread half the batter in the greased baking pan, sprinkle one cup of blueberries, layer remaining batter, and add the rest of the blueberries.

Sprinkle the chilled crumble topping evenly over the batter and blueberries.

Bake for 45-50 minutes until golden brown and a toothpick inserted in the center comes out clean.

Let it cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.