To make the green buttercream, whip the butter on high until light and fluffy, about 5 minutes. Lower the speed and slowly add in the powdered sugar, mixing until all has been added and the mixture is thick and well combined. Stir in the salt, vanilla, and heavy cream, and beat on high for about 8 minutes until thick and fluffy. Save about 1/2 cup of white buttercream, and set aside. Add a few drops of food coloring (as much as needed for a dark Frankenstein green and stir until the buttercream is evenly colored.

To make the chocolate buttercream, whip the butter on high until light and fluffy, about 5 minutes. Lower the speed and slowly add in the powdered sugar and cocoa powder, mixing until all have been added and the mixture is thick and well combined. Stir in the salt, vanilla, and heavy cream, and beat on high for about 8 minutes until thick and fluffy.

To assemble the cake, even out each layer by cutting off the tops if needed. Lay one layer cut side down, on a cake stand or platter. Add a scoop of green buttercream, and spread out evenly. Place another layer of cake on top, and repeat with another dollop of buttercream. Lay the third layer of cake on top. Coat the sides of the cake with green frosting and smooth it out with an offset spatula. You can put a very thin layer of green frosting on the top, but it isn't important. Save a bit of the green frosting for the nose.

Place the reserved white frosting in a piping bag, fitted with a round circle tip, and pipe out two eyes on the side of the cake, wherever the face will go. Place the reserved green frosting in a separate piping bag, fitted with a round tip as well, and pipe out a small nose in between his eyes.

Let's move on to his eye balls and other small details. Melt the chocolate in a microwave or a double boiler. Transfer the melted chocolate to a small piping bag, fitted with a small round tip or with a small opening that has been cut at the tip. Pipe out stitch on the side of his face, and then pipe out a mouth and two eye balls.

If you want to make the bolts on the side, pip them out free hand on a piece of parchment paper. Place in the freezer until firm. Then peel off the paper and stick one in each side.

To make his hair, transfer the chocolate buttercream to a large pastry bag fitted with a flat tip with ridges, it's usually a basket weave tip. Then pipe out long strands of hair over the entire top, making sure them come off over the edge. Then that's it! Your cake is done! Cut and serve or chill until ready to eat!