Beat cake mix, oil, buttermilk, eggs and dry pudding mix with electric mixer until fully combined. Fill each muffin liner 3/4 full with batter. Bake 15-18 minutes or until a toothpick can be inserted and removed clean. Transfer to rack and allow to cool completely before filling and frosting. Using a small sharp knife, cut a hole into the top of each cupcake and remove the cores.
Filling
Mix marshmallow fluff and frosting. Transfer to a quart sized plastic baggie and cut the tip off of one bottom corner. Pipe the filling into the cupcakes. Cool cupcakes in refrigerator while making the frosting.
Frosting
(Store bought chocolate can be used as a substitute)
Using electric mixer; beat butter, cocoa and vanilla. Add powdered sugar 1 cup at a time. Add milk if frosting is too stiff. Microwave frosting 15-20 seconds. Stir until smooth. Dip each cupcake into frosting glaze. Top with sprinkles. Store in refrigerator.
Notes
I think these cupcakes are best when eaten cold.Adapted from The Domestic Rebel