1. Preheat the oven to 350 degrees F and adjust the oven rack to the middle position. Spray or grease three 8-inch round cake pans and line with parchment paper. 
2. In the bowl of a standing mixer fitted with the whisk attachment or in a mixing bowl using a hand held mixer, whisk together the flour, cocoa powder, brown sugar, salt, and baking soda until well combined. 
3. In a separate bowl, whisk together the oil, vinegar, buttermilk, eggs, and vanilla. Incorporate half of this mixture into the dry ingredients and mix just until combined. Scrape down the sides and bottom of the bowl. Add the rest of the wet ingredients and mix just until combined. Add the coffee and mix on high speed just until smooth and fully combined.
4. Divide the batter evenly between the prepared cake pans and bake for 25 to 26 minutes or until a cake tester (knife or toothpick) comes out clean. Cool almost completely in the pan before turning each layer out, and cool completely before frosting with the buttercream (below).
5. To frost, place one of the cut cake halves onto a plate or cake stand and top with a generous amount of frosting. Smooth the frosting to the edges and then top with the unsliced cake layer. Top with more frosting and repeat until all three layer are secured. Slather lots of frosting all over the cake, over the top and down the edges and sides. Top with sprinkles, if you like. (This frosting is very creamy and light, so using dowels or toothpicks to secure the cake in place can be helpful – will keep it from sliding. Just be sure to remove them!)

For the Buttercream:

In the bowl of a stand mixer or in a mixing bowl with a hand-held mixer, add the heavy cream and whip on high speed until stiff peaks form. Add the cream cheese, cocoa powder, cooled coffee, and 2 cups of the powdered sugar. Whip on high speed until smooth and well-mixed. Taste and add more sugar to suit your sweetness preferences.