Preheat the oven to 375°F.Cream the butter and sugar until light. Add the egg, beating well. Add the vanilla.Sift together the flour, salt and baking powder, and add to the creamed mixture alternately with the milk: flour milk, flour, milk, ending on flour. Fold in the berries.Line a standard muffin tin with 6 liners, and fill equally with the batter. Sprinkle the sanding sugar over the tops of the muffins, and bake at for 30-35 minutes, or until the tops are golden and toothpick comes out clean.Remove muffins from tin, cool, and enjoy!