1Make and bake cupcakes as directed on cake mix box, using water, oil and eggs. Cool completely.
2In 1-quart saucepan, heat about 1 inch water to a simmer. Place larger-diameter heatproof glass bowl on top of saucepan. In bowl, place candy bars, butter and milk. Cook 10 to 12 minutes, stirring frequently, until candy bars are melted and mixture is smooth.
3Remove bowl from saucepan. Stir in vanilla. Add powdered sugar; beat with spoon until smooth. Cool 10 to 15 minutes.
4Place frosting in decorating bag with large round tip or resealable food-storage plastic bag with one bottom corner cut off. Squeeze bag to pipe frosting onto each cupcake; immediately top with 1 piece of candy bar.