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 mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.

3. In a separate medium bowl, whisk the egg until lightly beaten. Then, add the milk, sourdough discard, melted butter, and vanilla extract. Whisk until all the wet ingredients are thoroughly combined and smooth.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula or wooden spoon until just combined. Do not overmix; a few lumps are fine.

5. Gently fold in the blueberries. If using frozen blueberries, add them directly from the freezer to the batter.

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

7. Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the tops are golden brown and a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

8. Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.