1. 1. Prepare Your Baking Pan and Oven: Preheat your oven to 375 F (190 C). Lightly grease and flour an 8×8 inch square baking pan, or line it with parchment paper, leaving an overhang on two sides for easy lifting.

2. 2. Combine Dry Ingredients for the Cake: In a large mixing bowl, whisk together the 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and all ingredients are evenly distributed.

3. 3. Mix Wet Ingredients: In a separate medium bowl, whisk the egg, sourdough discard, milk, melted butter, and vanilla extract until well combined and smooth.

4. 4. Combine Wet and Dry Ingredients: Pour the wet ingredient mixture into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few small lumps are perfectly fine. Overmixing can lead to a tough cake.

5. 5. Prepare Blueberries and Fold In: In a small bowl, gently toss the blueberries with the 2 tablespoons of flour. This helps prevent them from sinking to the bottom of the cake. Carefully fold the floured blueberries into the cake batter until evenly distributed.

6. 6. Prepare the Crumble Topping: In a small bowl, combine the 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 cup brown sugar, and cinnamon. Add the cold, cubed butter. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

7. 7. Assemble and Bake: Pour the cake batter into your prepared baking pan and spread it evenly. Sprinkle the crumble topping generously over the batter. Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should be golden brown and the cake should feel set.

8. 8. Cool Before Serving: Let the cake cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving. This allows it to set properly and makes for cleaner cuts.