Preheat your oven to 325 degrees. Spray three 8-inch or four 6-inch round cake pans with non-stick cooking spray and then line with parchment paper. Spray again. Set aside.
In a medium-sized bowl, combine the graham cracker crumbs, melted butter, and sugar. With a rubber spatula, stir until all the graham cracker crumbs are combined with the melted butter. You should be able to squeeze the mixture and have it stick together.
Divide the mixture evenly between the three pans and press down firmly with your hand or the back of a spoon (about ¾ cup in each pan).
Bake for 10 minutes. Let cool while you make the cake layers.
For the Graham Cracker Cake
Keep your oven at 325.
If you need to pulverize your graham crackers, do it now. They should resemble sand.
In a medium-sized bowl, whisk the graham cracker crumbs, flour, baking powder, cinnamon, nutmeg, baking soda, and salt. Whisk until the ingredients are well-blended. Set aside.
In the bowl of a stand mixer fitted the paddle attachment, beat the butter, oil, and sugar on medium-high speed until light and fluffy, about 3-4 minutes. Add the eggs, one at a time, beating well after each addition and making sure to scrape down the sides of the bowl between each addition.
With the mixer on the lowest speed, add ⅓ of the dry ingredients and mix just until combined. Slowly add half of the buttermilk, followed by another ⅓ of the dry ingredients, the second half of the buttermilk, and the final ⅓ of dry ingredients. Mix until combined, about 30 seconds. Scrape down the sides of the bowl and slowly mix for another 20-30 seconds.
Divide the batter evenly among your pans (about 20 ounces in each-for 8 inch pans) right on top of the baked graham cracker crust and bake for about 30 to 35 minutes, or until a toothpick inserted in the center comes out with only a few moist crumbs on it. Cool 15 minutes on a wire rack before transferring the cake layers out of the cake pan and onto cooling racks to cool completely.
For the chocolate buttercream
In a microwave safe bowl, heat the chocolate for 30 seconds, stir, heat for 30 more seconds, and set aside. The chocolate will not be melted all the way, but this will help it melt quicker when you add the cream.
In another microwave proof bowl, heat the cream until nearly boiling. Make sure it doesn’t boil over. Pour the hot cream over the slightly warm chocolate chips. Let it sit for 2 minutes and then stir. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter for about 2 minutes on medium-high speed.
Add the chocolate/cream mixture. Scrape down the sides and bottom of the bowl with a rubber spatula and beat for 1 more minute.
Add the powdered sugar, vanilla, and salt. Mix on low until combined, then turn the mixer to a medium high speed, and beat for 3 minutes.
With a wooden spoon or a rubber spatula, beat by hand to mix out any air pockets.
Assembly
Place the first cake layer on a cake board, with the graham cracker layer on the bottom. Gently and evenly spread ¼ cup of the chocolate buttercream over the cake layer. Pipe a rim of chocolate around the edge of the cake.
Spread about ¾-1 cup of the chocolate buttercream on the top of the cake layer.
Repeat with the second layer of cake.
Place your final cake layer, with the graham cracker layer on the bottom, on top of the second layer of filling.
Apply a thin layer of the chocolate buttercream around the entire cake and freeze the cake for 10 to 15 minutes.
Once the crumb coat is set, finish frosting and decorating the cake with the remaining chocolate buttercream. Serve at room temperature.