Preheat oven to 350F. Grease 24 mini muffin pan with cooking spray. In a large bowl, whisk together flour, sugar, baking powder, baking soda and salt. In another bowl, whisk together egg, milk, vegetable oil, and vanilla. Add the wet ingredients into the dry ingredients and stir until the mixture is well combined. Scoop rounded tablespoons of batter into the greased muffin tins, about full. Bake until a toothpick inserted into the center of the muffin comes out clean, about 12 to 15 minutes. Allow to cool slightly in pan before removing them from the pan. Whisk together sugar and cinnamon in a bowl. Dip each muffin in the melted butter then toss in cinnamon-sugar mixture until well coated.