1. Prepare your baking dish and preheat the oven: Start by preheating your oven to 375 F (190 C). Grease and flour an 8×8 inch square baking pan, or line it with parchment paper for easy removal. This prevents sticking and ensures a clean release of your cake.

2. Combine dry ingredients: 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 and the ingredients are evenly distributed. This step ensures your leavening agents are well mixed for an even rise.

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

4. Combine wet and dry mixtures: Pour the wet ingredient mixture into the dry ingredient mixture. 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. Fold in the blueberries: Gently fold in the blueberries until they are evenly distributed throughout the batter. If using frozen blueberries, do not thaw them; add them directly to the batter. This helps prevent them from sinking to the bottom.

6. Transfer to pan and top: Pour the batter into your prepared 8×8 inch baking pan and spread it evenly. Sprinkle the remaining 2 tablespoons of granulated sugar over the top of the batter. This creates a lovely slightly crisp, sweet crust.

7. Bake the cake: 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 top should be golden brown.

8. Cool and serve: Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving. This allows the cake to set properly and prevents it from crumbling.