In a large bowl combine flour, sugar, salt and baking powder. Set aside. In a separate bowl, combine milk, egg and vanilla, mix well to combine. Stir melted butter into liquid mixture. Mix until well combined. Add the liquid ingredients to the dry ingredients and stir until well combined, but do not over mix. Due to the baking powder, the mixture will fill with tiny air bubbles and can appear a bit foamy, this is okay, it's what helps puff up the pancakes.