1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it well with butter or non-stick spray.

2. In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and everything is 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 bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix the batter; a few lumps are perfectly fine.

5. Gently fold in the blueberries. If using frozen blueberries, do not thaw them first. Stir just enough to distribute them throughout the batter without crushing them.

6. Divide the batter evenly among the 12 prepared muffin cups. Fill each cup about two-thirds full.

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.

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