In the bowl of a standing mixer (or large bowl if you are using a hand mixer) cream butter and shortening together. Slowly add powdered sugar, and milk.
Beat ingredients until combined and creamy in texture. Add the marshmallow fluff , continue to beat until incorporated and a good spreading consistency.
You can pipe on with a decorating tip in a piping bag (I used tip 1M) or spread with spatula. You can also scoop on with an ice cream scoop!
Top each with graham cracker pieces/crumbs and one little portion of chocolate bar.