Well here's the recipe, then I'll give tips later.
2 sticks unsalted butter, softened
1 c confectioner's sugar
8 oz of Marshmallow Fluff
Cream butter with 1/3 sugar. Add in remaining sugar and mix until creamy. Fold in Marshmallow Fluff. See simple. :)
Well yesterday, not so simple. Here are some tips that I found. Jet Puffed Marshmallow may taste good, it doesn't work well in this recipe for some reason. I like to you Marshmallow Fluff brand. It seems to be more stiff and stands up better with the cupcakes.
If you want more of a marshmallow flavor, just add more. It will increase the amount made, but won't really hurt anything. I'm not sure if this really was a factor, but it was different yesterday than the other two times I made this; the weather. This may be an icing that is just meant for cool or cold weather. It was warm and almost humid yesterday when Mom and I were making this. So not sure, but a possibility.
Hope these tips help and you like the icing as much as I do! Happy Baking :)