In a small bowl, combine the flour, sugar, baking powder and salt. In another bowl, whisk the egg, milk, butter and vanilla; stir into dry ingredients just until moistened. Fold in blueberries., Fill greased or paper-lined muffin cups three-fourths full. Bake at 400° for 18-22 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before removing from pan to a wire rack. Serve warm.