Bake the Cake: Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans with butter or non-stick spray, then set aside. In a large mixing bowl, combine 2 1/2 cups all-purpose flour, 2 cups sugar, 1 cup cocoa powder, 1 1/2 tsp baking powder, 1 1/2 tsp baking soda, and 1 tsp salt. Whisk the dry ingredients until well blended. Next, add 3 large eggs, 1 cup buttermilk, 1/2 cup vegetable oil, and 2 tsp vanilla to the dry mixture. Mix until combined.
Add Hot Water: Gradually stir in 1 cup hot water. This step may make the batter quite thin, but that's perfectly fine! It helps to keep the cake moist. After the water is fully incorporated, pour the batter evenly into your prepared cake pans.
Bake the Cakes: Place the pans in the oven and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Be sure to keep an eye on them towards the end of baking. Once done, remove from the oven and let them cool in the pans for about 10 minutes. Carefully transfer the cakes to a wire rack to cool completely.
Prepare the Frosting: In a mixing bowl, beat together 1 cup butter (softened) and 8 oz cream cheese until smooth and fluffy. Gradually add in 4 cups powdered sugar and 1 tsp vanilla. Mix on low speed until combined, then increase to medium-high speed and whip until the frosting is light and fluffy.
Assemble the Cake: Once the cakes are completely cool, place one layer on a serving plate. Spread a generous amount of frosting between the two layers. Top with the second layer and apply a crumb coat around the entire cake. Chill the cake in the refrigerator for about 30 minutes to set the frosting.
Frost and Decorate: After chilling, apply the final layer of frosting on the outside of the cake. Smooth it out for an elegant finish. Decorate with dark chocolate shavings on top, and sprinkle edible glitter for an optional luxurious touch. Avoid the glitter if you prefer a more classic look.