1. 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. In a large mixing bowl, whisk together 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt until well combined.

3. In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, egg, and vanilla extract until smooth.

4. Pour the wet ingredients into the dry ingredients. Stir gently with a spatula until just combined. Be careful not to overmix. A few lumps are perfectly fine.

5. In a small bowl, toss the fresh blueberries with the remaining 2 tablespoons of all-purpose flour. Gently fold the floured blueberries into the cake batter.

6. In a small bowl, combine 1/2 cup all-purpose flour, 1/4 cup granulated sugar, and 1/4 cup packed light brown sugar. Add the cold, diced butter. Using your fingertips or a pastry blender, work the butter into the flour mixture until coarse crumbs form.

7. Pour the cake batter into the prepared baking pan and spread it evenly. Sprinkle the crumb topping generously over the batter.

8. Bake for 30-35 minutes, or until a wooden skewer inserted into the center of the cake comes out clean. The crumb topping should be golden brown. Let the cake cool in the pan on a wire rack for at least 15 minutes before slicing and serving.