Preheat the oven to 350°F and line a muffin tin with four cupcake liners.
In a mixing bowl, stir together the flour, sugar, baking powder, and salt.
Add the melted butter, egg, milk, and vanilla extract. Mix until the batter is smooth and no dry streaks remain.
Spoon half the batter evenly into the four liners. Drop one teaspoon of Halloween sprinkles into the center of each cupcake.
Cover the sprinkles with the remaining batter and smooth the tops lightly.
Bake for 18 to 20 minutes until a toothpick inserted in the center comes out clean.
Let the cupcakes cool completely in the pan, then spread vanilla frosting over each one and finish with extra sprinkles on top.