Preheat the oven to 350ºF and prepare three 6-inch or two 8-inch cake pans by spraying the sides with cooking spray and fitting the bottoms with a wax paper or parchment cake circle.

Place all of the dry ingredients into the bowl of a stand mixer and stir on low for 30 seconds to fully combine them. Add the vegetable oil, eggs, vanilla, and buttermilk and mix on low until just combined. With the mixer still on low, add the hot water in a slow stream, then turn the mixer to medium and beat until smooth, about 2 minutes. The batter will be very thin.

Pour into prepared cake pans no more than ⅔ full and bake for 32-36 minutes, until a toothpick inserted comes out clean. Cool completely before frosting.

In a stand mixer fitted with a paddle attachment, whip the butter on medium speed until creamy and light in color, about 4 minutes. Scrape down the bowl and paddle, then add half of the powdered sugar. Mix on low speed until just incorporated, then add the rest of the powdered sugar. With the mixer still in the off position, sift in the black cocoa powder and add the vanilla, milk, and salt. Mix on low until all ingredients are thoroughly combined, about 2 minutes, scraping down the bowl and paddle as needed.

The buttercream will look dark brown at this point, but the color will darken significantly over the next several hours. See the recipe notes for storage instructions if you're not planning on frosting the cake the same day.

Once the cake layers have cooled completely, level them to your desired height. Fill and stack the layers with black buttercream, then crumb coat the cake with more black buttercream. Refrigerate the crumb coated cake for 20-30 minutes to let the frosting firm up.

To create the design pictured, use the black buttercream to create a smooth finish on the cake, then refrigerate it for at least 30 minutes. Use the rest of the black buttercream to create lambeth piping. Accent the lambeth piping with round black sprinkles where desired.