In a small saucepan, add beer and brown sugar. Stir well. Cook on medium-low heat for about 30 minutes, stirring frequently. You want to reduce your mixture to a syrup like consistency. It will be runny. Remove from heat and let set for five minutes. The mixture should be almost like a molasses.

In a mixing bowl, add marshmallow cream and frosting. Pour in syrup. Beat until combined well.

Place frosting into a piping bag and pipe onto cupcakes. Then top them with sprinkles!