Line muffin pan with baking cups. Fill 1/2 to 2/3 full with your favorite cupcake recipe or batter from a mix. Bake according to recipe instructions. Cool completely.
Spatula ice tops of cupcakes with white buttercream icing. Tint remaining buttercream icing red, blue, yellow or other favorite colors. Pipe tip 8 or 12 ball balloons. Holding tip slightly above surface, squeeze the bag, applying a steady even pressure. As the icing begins to build up, raise the tip with it, but keep the tip buried in the icing. Stop squeezing, bring the tip to the surface and use edge of tip to shave off any icing point.
Position tip 2 at the base of balloon. Pipe a dot for the balloon neck, then pull out balloon strings in matching colored icing.