Place the first cake layer on a cake plate. Pipe a thick ring of nutella buttercream around the edge of the cake. Pipe and fill the marshmallow mixture into the center of the cake. Sprinkle about ¾ cup of graham cracker crumbs over the marshmallow and nutella buttercream. Place the second cake layer and repeat the above steps. Place the third cake layer on top. Using the nutella buttercream, coat the outside and top of the cake. Decorate the cake with any remaining graham cracker crumbs and marshmallow. If you have a kitchen torch, toast any marshmallow on the top of the cake.