Cake: Prepare two or three chocolate cake layers (using a classic cocoa cake batter). Bake and cool. Filling: Beat softened butter with marshmallow cr?me, powdered sugar and vanilla until fluffy (this mimics whoopie pie filling). Assemble: Spread marshmallow filling between the chocolate cake layers. If three layers, also spread on top of middle layer. Finish with chocolate frosting or glaze on top of cake.