1. Preheat your oven to 400 F (200 C). Line a 12-cup muffin tin with paper liners or grease it well with butter or non-stick spray.

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 and the ingredients are evenly distributed.

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

4. Pour the wet ingredients 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 lead to a tender muffin.

5. Gently fold in the blueberries. If using fresh, make sure they are washed and dried. If using frozen, add them directly from the freezer. Distribute them evenly throughout the batter.

6. Divide the batter evenly among the 12 prepared muffin cups. Each cup should be about two-thirds full. Sprinkle the remaining 1 tablespoon of granulated sugar over the tops of the muffins.

7. Bake for 20-25 minutes, or until the tops are golden brown and a toothpick inserted into the center of a muffin comes out clean. The muffins should feel springy to the touch.

8. Let the muffins cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely.