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 the all-purpose flour, ½ cup granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

3. In a separate medium bowl, whisk together the sourdough discard, milk, melted and cooled butter, large egg, vanilla extract, and lemon zest until well combined. The mixture should be smooth and uniform.

4. 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 lumps are perfectly fine and will result in a more tender cake. Overmixing can lead to a tough texture.

5. Gently fold in the fresh or frozen blueberries. If using frozen, do not thaw them; simply add them straight from the freezer. Distribute them evenly throughout the batter.

6. Pour the batter into your prepared baking pan and spread it evenly. Sprinkle the remaining 1 tablespoon 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 spring back lightly when touched.

8. Remove the cake from the oven and let it cool in the pan on a wire rack for at least 10-15 minutes before slicing and serving. This allows the cake to set properly.