Preheat the oven to 350 degrees Fahrenheit.
Prepare either 1 9x13 cake pan or 2 round 8 inch cake pans. Spray the bottom(s) with oil then add parchment paper just to the bottom. I like to use these parchment rounds to make it easy. Set aside. In a large mixing bowl, whisk together all wet ingredients aside from the coffee/boiling water.
Whisk in the sifted dry ingredients.
Pour in the coffee or boiling water and whisk until combined. The batter will be fairly thin.
Add the batter to the prepared cake pan. If making a layer cake, try to measure each cake as equally as possible, or use a kitchen scale for best results. Bake for 35-45 minutes for a sheet cake, 32-37 for layer cake, or until a toothpick comes out clean. All ovens are different so just keep an eye on it.
Let cool in the pan(s) for about 10-20 minutes, then carefully flip onto a cooling rack. They should slide right out, then peel off the bottom parchment paper and flip back so they're upright.
Let cool completely before frosting.
To make the buttercream, add softened/room temperature butter to a bowl of a stand mixer or a large bowl with a handheld mixer and beat until smooth and creamy.
Beat in the sifted cocoa powder.
Beat in the sifted powdered sugar, one cup at a time until your desired consistency is reached. Finally, beat in the milk and vanilla until well combined.
Frost the cake and add sprinkles if desired. Enjoy!