In a medium bowl, add white and brown sugar, flour, cinnamon, and cold, cubed butter.
Crumble the larger pieces of butter with your fingers and mix by hand until the butter is well coated with the flour mixture.
Place in the fridge to chill until ready to top the sourdough blueberry buckle.
Preheat your oven to 375 degrees f.
Grease the inside of a round cake pan or 9x9 square pan with butter or cooking spray. Then line it with parchment paper, pressing it down so that all the edges are as smooth as possible. Grease the inside of the parchment paper as well.
In a large bowl, whisk together 1 ½ cups flour, 2 ½ teaspoon baking powder, and ½ teaspoon salt. Set aside.
In the bowl of an electric mixer with the paddle attachment, cream ¾ cup sugar and ½ stick of softened butter until it is light and fluffy.
Then add a large egg and mix until pale yellow color.
In a medium bowl, mix together ½ cup sourdough starter, ½ cup milk, 1 tablespoon of lemon zest, and ½ tsp of pure vanilla extract.
Add the wet ingredients to the stand mixer and mix on low speed for about a minute, stopping to occasionally scrape the bottom and the sides of the bowl so that everything is incorporated.
Add the dry ingredients to the wet ingredients, half a cup at a time while mixing on low speed. Be sure not to over-mix the batter.
In a large bowl, pour flour over the fresh or frozen blueberries. You shouldn't need much flour, just enough to coat the blueberries. Mix with a wooden spoon until they are evenly coated.
Scoop half of the batter into the prepared pan and spread smoothly. Pour 1 cup of blueberries on top of the batter. Place the remaining batter on top and spread evenly. Add the remaining blueberries on top of the cake.
The finishing touch is to sprinkle an even layer of the crumb topping - don't miss the corners!
Place your pan into the preheated oven and bake until the 45-50 minute mark or until golden brown. Double-check that it is fully baked with the toothpick test. If the toothpick comes out clean, it is done baking.
Remove the sourdough blueberry buckle from the baking dish and cool on a wire rack.
Once it is just warm to the touch, slice it up in wedges or squares and display them on your favorite serving plate!