Begin by sifting together flour, baking powder, salt, and sugar into a large bowl. You could also use a whisk for this. Set aside. In a separate bowl, beat the egg slightly, then add the milk and melted butter and whisk together. Pour the egg and milk mixture into the dry ingredients. Stir together until dry ingredients are just moistened. The batter should be lumpy. Fold in the berries.
Grease 12 muffin cups, but on the bottom. (This technique makes for a muffin that is more rounded on the top.) Fill each of the muffin cups 3/4 full. Bake at 400 degrees for 20-25 minutes until golden brown. Test with a toothpick to make sure it comes out clean. Remove the muffins from the baking pan and cool on a wire rack. Makes 12 muffins.
I use frozen blueberries if I have them and I have used other berries as well - raspberries, blackberries.