Make cupcakes according to box instructions
Beat butter in a mixing bowl or stand mixer until creamy
Add in confectioners sugar a little bit at a time until all is added and blended
Add in vanilla and marshmallow blend well
After cupcakes have cooled cut a small cone in to the top of your cupcake with a knife. This is very simple (see picture). Remove the cut piece of cupcake and set to side.
Place your frosting in a plastic bag or in a pastry bag and squeeze into the hole you just created
Trim your piece of cupcake that you removed and place back on top.
Swirl your frosting on top of your cupcake.
Garnish with mini marshmallows, mini chocolate chips and straw.