Preheat oven to 350 degrees F. Line a cupcake pan with 6 paper liners.
In a large mixing bowl, add 3/4 cup flour, 3/4 teaspoon baking powder and 1/4 teaspoon salt. Mix with fork.
In another bowl, add the 4 Tablespoons melted butter, 6 Tablespoons sugar, 1 egg, 1/4 cup milk, and 1 teaspoon. Whisk with a fork.
Pour the bowl of dry ingredients into the bowl of wet ones. Whisk. Fold in 3 Tablespoons of sprinkles.
Evenly pour the cupcake batter into the 6 liners. Bake for 19-24 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cupcakes fully before frosting.
To make the frosting, add the 1/2 cup of room temperature butter, 2 teaspoons vanilla, 2 cups of powdered sugar, and 2 Tablespoons of milk to a food processor and mix. Add food coloring to color your frosting if you would like.
Frost cupcakes and top with rainbow sprinkles.