1. Prepare Your Baking Pan and Oven: Begin by preheating 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. This prevents sticking and makes cleanup simpler.

2. Combine Dry Ingredients: In a large mixing bowl, whisk together the 1 cup of all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed. This step ensures proper leavening throughout the cake.

3. Mix Wet Ingredients: In a separate medium bowl, whisk together the active sourdough discard, milk, melted butter, egg, and vanilla extract until well combined and smooth. The sourdough discard should be at room temperature for easier mixing.

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, as this can lead to a tough cake. A few small lumps are perfectly fine.

5. Prepare and Fold in Blueberries: In a small bowl, gently toss the fresh blueberries with the extra tablespoon of all-purpose flour. This light coating helps prevent the blueberries from sinking to the bottom of the cake during baking. Carefully fold the floured blueberries into the cake batter until they are evenly distributed.

6. Bake the Cake: Pour the batter into your prepared 8×8-inch baking pan and spread it evenly. Place the pan in the preheated oven and bake for 25-30 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.

7. Cool Before Serving: Once baked, remove the cake from the oven and place it on a wire rack to cool in the pan for about 10-15 minutes. This allows the cake to set before you attempt to remove it. After partial cooling, you can lift the cake out of the pan using the parchment paper overhang and let it cool completely on the wire rack before slicing and serving.