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 removal.

2. In a large mixing bowl, whisk together the all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps.

3. In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, egg, vanilla extract, and lemon zest until well combined and 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 okay. Overmixing can lead to a tough cake.

5. Gently fold in the blueberries. If using fresh, they can go straight in. If using frozen, toss them with a tablespoon of flour from the measured amount before adding to the batter to prevent them from sinking.

6. Pour the batter into the prepared baking pan and spread it evenly. Sprinkle the remaining 2 tablespoons of granulated sugar over the top of the batter. This will create a lovely slightly crisp, sweet crust.

7. 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 edges slightly pulled away from the sides of the pan.

8. 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 slices.