1. Preheat your oven to 375 F (190 C). Lightly grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan.

2. In a large mixing bowl, combine 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt. Whisk thoroughly to mix and break up any lumps.

3. In a separate medium bowl, whisk together the melted butter, sourdough discard, 1/2 cup milk, egg, and vanilla extract until well combined and smooth.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir with a spoon or spatula just until the flour streaks disappear. Do not overmix; a few lumps are fine.

5. Gently fold in the blueberries. If using frozen, add them directly from the freezer. Distribute them evenly throughout the batter. Pour the batter into your prepared baking pan and spread it out evenly.

6. For the streusel topping: In a small bowl, combine 1/2 cup all-purpose flour, 1/4 cup granulated sugar, and brown sugar. Add the cold, diced butter. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Sprinkle this streusel mixture evenly over the top of the cake batter in the pan.

7. Place the pan in the preheated oven and bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The streusel should be golden brown and the cake should feel firm to the touch.

8. Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for at least 10-15 minutes before attempting to remove it. If adding a glaze, whisk together the powdered sugar and 1-2 tablespoons milk or lemon juice until smooth. Drizzle the glaze over the slightly warm or cooled cake. Slice and serve!