Preheat oven to 350 degrees. Lightly grease two 9-inch cake pans. Cut out two circles of parchment and fit one to the bottom of each cake pan.
Chocolate cake layer: In a large bowl, beat the oil, brown sugar, and sugar for 2-3 minutes until combined. Whip in the eggs, one at a time, then the vanilla and yogurt. Add the flour, cocoa powder, black cocoa powder, baking powder, baking soda, and salt. With the mixer running, stream in the buttermilk until the batter is just combined. Do not overmix. Pour into the first cake pan.
Pumpkin cake layer: In a large bowl, beat the oil, brown sugar, and sugar for 2-3 minutes until combined. Whip in the eggs, one at a time, then the vanilla, yogurt and pumpkin. Add the flour, cinnamon, ginger, cloves, baking powder, baking soda, and salt. Mix until just combined. Do not overmix. Pour into the second cake pan.
Bake both layers for 30-35 minutes until a toothpick inserted into the center emerges with just a few moist crumbs.
Cool the cake layers in the pan for 10 minutes, then run a knife around the edges to loosen and invert each layer onto a wire rack. Peel off the parchment from the bottoms. Cool completely.
Dark chocolate buttercream: Whip the butter and vanilla extract on medium speed until the butter turns pale and creamy, about 2-3 minutes. Add the cocoa, black cocoa, 5 cups of the powdered sugar, and 3 tablespoons of cream. Whip on medium speed until fluffy. You may need an extra tablespoon of two of cream to achieve the desired texture. Scrape down the sides of the bowl with a spatula, as needed, to properly incorporate all the ingredients.
Set one cake layer onto a cake stand or flat plate. I placed the chocolate layer on the bottom. Frost generously overtop. Set the pumpkin layer on top and frost atop and around to thoroughly coat. Use your favorite piping tip with decorate the perimeter of the top layer. I did some cute little swirly curls with a Wilton 1M.
Orange buttercream: Whip the butter and vanilla until creamy and combined, about 1-2 minutes, then add the powdered sugar and continue mixing. Follow package directions on your dyes (and, honestly, go by vibes) to create your ideal orange hue. I used an assload of egg dye for mine, but if you're using gel, you will probably need less since its more vibrant.
Pipe pumpkin shapes along the lower perimeter of the cake. They don't need to be perfect! Make "stems" with some of your leftover chocolate frosting. SO CUTE.