Preheat oven to 350℉.
Prepare muffin tin by greasing cups or using paper liners.
In a large bowl, stir together the flour, baking powder, baking soda, salt, and the sugars.
In a small bowl, add starter, milk, egg, melted butter, and vanilla and whisk together.
Now add the sourdough mixture to the dry ingredients. Whisk until well combined.
Toss frozen blueberries in 2 Tablespoons of flour and mix just until combined. If you're using fresh blueberries, just toss them in.
Divide the batter evenly between the 12 prepared muffin cups.
Bake in preheated oven for 20-25 minutes until browned on top.
After baking, remove them from the muffin pan, split them open, and butter them.
Put them in a cloth lined basket to keep them warm for serving. If you are going to serve them later, just put them on a wire rack to cool.