Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
Mix Dry Ingredients: In a mixing bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt until well combined.
Add Wet Ingredients: Add the eggs, buttermilk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for about 2 minutes.
Incorporate Water: Stir in the boiling water until the batter is well combined; keep in mind that the batter will be thin.
Pour Batter: Evenly pour the batter into the prepared cake pans.
Bake: Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Cool: Allow the cakes to cool in the pans for about 10 minutes, then remove them from the pans and transfer to wire racks to cool completely.
Frost the Cake: For the chocolate frosting, beat the softened butter until creamy. Gradually add the powdered sugar and cocoa powder, mixing thoroughly. Add vanilla and enough milk to reach your desired consistency.
Assemble: Place one cake layer on a serving plate, spread frosting on top, and then add the second layer. Frost the top and sides of the cake.
Serve and Enjoy: Slice, serve, and indulge in the luxurious flavor of your homemade Sublime Black Velvet Treat!