In a large mixing bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in vanilla. Combine the flour, baking powder and salt; add to the creamed mixture alternately with milk, beating after each addition. Fill cupcake liners 2/3 full and bake in 350-degree oven for 25 minutes. Let cool.
In a large mixing bowl, combine all three ingredients and mix well. Cut the center chunk out of each cupcake and set aside. Fill cavity in cupcake with a large dollop of frosting. In the front of the cavity, stand a gummy eyeball. Decorate with halloween sprinkles and colored sugar. Cover leftovers loosely and refrigerate.