Heat over to 375 degrees. Line 12 muffin cups. Beat butter with mixer until creamy,add sugar and beat, add eggs and beat into mixture. Beat in vanilla and baking powder. With rubber spatula fold in half the flour,then half the milk. Repeat with remaining flour and milk.. Add frozen blueberries..mix. Spoon batter into muffin cups. Sprinkle tops with granulated sugar. Bake 25-30 minutes until golden brown and top is springy to touch.