For the cake: Put a rack in the middle of the oven and preheat the oven to 350 degrees F. Butter 2 (9 by 2-inch) nonstick round cake pans. Line the bottom of each with a round of parchment paper, and then butter the paper.
Sift the flour, cocoa, baking soda and salt together onto parchment paper. With an electric mixer on medium-high speed, cream the butter and sugars until light and fluffy, 4 or 5 minutes. Add your eggs, 1 at a time, mixing until completely incorporated before adding each additional egg. Mix in the chocolate and vanilla, making sure to combine really well. Turn the mixer to low and slowly add the dry ingredients, alternating with the buttermilk, in 3 batches, starting and ending with the flour mixture. Make sure to let it mix thoroughly.
Pour half the batter in each pan and spread evenly with a spatula or butter knife. Bake until a toothpick comes out clean, about 25 minutes. Remove the pans to racks and allow the cakes to cool for 10 minutes or so. Loosen the edges of the cake from the pan with a knife, then turn them onto the racks. Remove the paper and let the cakes completely cool.
To assemble: Put some Milk Chocolate Buttercream on top of 1 cake layer, pushing out to the sides a bit. Sprinkle some ground up chocolate wafers on top. Place the other cake layer on top and flip it over so the flat surface is up. Ice with the rest of the buttercream. Don't try to make it perfectly smooth, the more bumpy it is the better, we're creating a graveyard!
With your black gel, write out RIP on the chocolate sandwich cookies to make them look like gravestones.
Sprinkle more ground wafers on top of the cake to look like dirt. Pat some more around the sides. Take your chocolate tree and put right through the cake in the back. Add some gravestones and some pumpkins. And the chocolate bones, pressing into the sides like an X. Arrange chunks of rock candy around.
Combine the butter, salt and 2 cups of the confectioners' sugar in a mixer with the whisk attachment. Beat until light and fluffy. Add the melted chocolates, vanilla and the remaining 3 cups sugar. Whisk on medium-high until smooth and fluffy.
Put the chips in a heatproof glass bowl over a pot of simmering water. Stir to melt. Fill a zip top bag with the melted chocolate and pipe it out on a baking sheet lined with foil in the shape of a tree. Let cool.
Put the chips in a heatproof glass bowl over a pot of simmering water. Stir to melt. Add the melted butter to the chips to make the mixture more workable. Take your pretzel stick and push a marshmallow on each side. Coat in the white chocolate, and let dry on parchment paper.