Preheat oven to 375 F. Butter and flour your muffin pan (or you could use baking spray which is better for muffin pans). Add the flour and baking powder to a bowl and whisk together.
In a separate bowl mix the cooled butter, milk, egg, and brown sugar. Add the flour to the wet ingredients. Mix in the dollop of sour cream. Fold in the blueberries.
Fill 6 large muffin cups, (they will be heaping) and sprinkle with sugar. Bake for about 20-25 minutes or until toothpick in center comes out clean. They should be a nice golden brown.