1. Prepare Your Muffin Tin: Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with butter or non-stick spray. This prevents sticking and makes cleanup a breeze.

2. Combine Dry Ingredients: 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 the ingredients are evenly distributed.

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

4. Combine Wet and Dry: 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 lumps are perfectly fine. Overmixing can lead to tough muffins.

5. Fold in Blueberries: Gently fold in the fresh or frozen blueberries. If using frozen, add them directly from the freezer to prevent them from sinking to the bottom of the muffins.

6. Fill Muffin Cups: Divide the batter evenly among the 12 prepared muffin cups. You can use an ice cream scoop for easy and consistent portioning. Fill each cup about two-thirds full.

7. Bake Until Golden: 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. Cool and Serve: Let the muffins cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm or at room temperature.