Preheat your oven to 350°F/175°C. Grease and line three 8" round cake pans with parchment paper. If you only have 2 pans, line those and once they're out of the oven and slightly cool, regrease and reline one of the pans to bake the third layer.
In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, black cocoa powder, sugar, baking powder, baking soda and salt.
3 cups all purpose flour, 1 cup black cocoa powder, 1 ½ teaspoon baking powder, 3 teaspoon baking soda, 3 cups sugar, 2 teaspoon kosher salt
In a large jug, whisk the eggs, oil, buttermilk and vanilla until smooth.
3 large eggs, ¾ cup vegetable oil, 1 ½ cups buttermilk, 1 teaspoon vanilla extract
With the mixer running on low, slowly pour in the wet ingredients. Note, this is a lot of batter so it might get messy! Once all the wet ingredients are in, stop the mixer and scrape down the sides and bottom of the bowl to make sure there are no dry spots left. Start the mixer again on the lowest speed and pour in the hot water.
1 ½ cups boiling water
Pour the cake batter into the prepared pans, it should be about 680 grams of batter per pan.
Bake for 35-40 minutes. A toothpick inserted should come out mainly clean, there might be a few crumbs. Let the cakes cool for 15 minutes in the pan, then invert the pans on a wire rack and let the cakes cool completely on a wire cooling rack.
Make Frosting
In a stand mixer fitted with the paddle attachment, beat the butter and confectioners' sugar together on low speed until well combined.
Increase the speed to medium-high and beat for a 2 minutes until it becomes very thick.
Stop the mixer and add the salt, vanilla, and cocoa powder into the bowl. Begin by beating slowly, then increase to medium-high speed for another 2 minutes
2 teaspoon vanilla extract, ½ teaspoon kosher salt, 1 ½ cups black cocoa powder
Turn the mixer to your lowest speed and pour in the heavy cream. Beat until it becomes very creamy and spreadable.
½ cup heavy cream
I used a Wilton 1M piping tip to frost the top of the cake!
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way.Room Temperature: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking. Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your frosting much creamier and richer.Salt: If you don't have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Cocoa Powder: In the images here, I've used black cocoa powder in the frosting and regular dutch process cocoa powder in the cake. If you want your cake to be truly black, then use an equal amount of black cocoa powder in the cake. I recommend the brands De Zaan or Van Houten. If you don't have black cocoa powder then you can substitute an equal amount of dutch-process cocoa powder, but please note that it won't be as dark so you might want to add a bit of black food coloring to the frosting. Storage: Store the cake in the fridge for up to 5 days. Let them sit at room temperature for 30 minutes before eating.