To make the frosting:Â In a large bowl, or the bowl of a standing mixer, beat the butter and marshmallow until light and fluffy, about 5 minutes. Add the sugar, beating until just combined. Stir in 1 tablespoon of cream and the vanilla, then beat on high speed for 4 minutes. If the frosting is too thick, add more cream, up to 1Â tablespoon
Assemble:Â Fill a disposable decorator bag, fitted with a cupcake filling tip (or large writing tip, I use a Wilton #7), with the chilled filling. Insert the tip into the center of the cupcake and squeeze to fill. The cupcake is full when you can feel the filling push back against the tip (it will slowly force the decorating tip out of the cupcake).