She made the cupcakes using Bob's Red Mill Chocolate Cake Mix and then piped in a clone of the Hostess cupcake cream using a piping bag with a large star tip. (Any large tip would work fine, though.) Here's the cream recipe. Give it a try!
Cupcake Cream Filling
Beat together 1 c. shortening, 1 1/2 c. confectioner's sugar and 2 c. Marshmallow Fluff (or marshmallow cream). Add in 1 1/2 t. vanilla and mix until blended.