Preheat oven to 350°F/175°C. Grease 3 8-inch cake pans with butter, then line the bottoms with parchment paper.
Combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt in the bowl of a stand mixer, using the paddle attachment. Set aside. In a large jug, combine the buttermilk, oil, eggs and vanilla.
While the mixer is on low speed, slowly pour in the wet ingredients. While mixer is till running, slowly add the coffee and mix. Your batter will be very thin.
Pour the batter into the prepared pans. Bake for 35-40 minutes, until a toothpick comes out clean when inserted.
Allow the cakes to cool completely before removing from pans.
Swiss Meringue Frosting
Fill a medium saucepan with an inch of water and bring to boil. Combine the sugar, salt, cream of tartar and egg whites in a large mixing bowl (if you have a stand mixer, you'll want to use that bowl), set the bowl above the simmering water.
Heat, whisking constantly, until the sugar dissolves. The mixture will go from a yellowy, granular substance to white and opaque, this takes about 5 minutes. Using a thermometer, it should read 115°F. If you don't have a thermometer, when you pull the whisk up feel the mix with your fingers – if it is no longer grainy then it is ready for the next step.
Put the bowl into your stand mixer with a whisk attachment and beat on medium-high until you get a stiff, glossy meringue – about 8-10 minutes. You'll want to be beating the mixture until the bowl is completely cool.
Add butter one piece at a time, allowing it to incorporate fully, while still at medium-high speed. This should take 8-12 minutes. When you've added all the butter and the mixture looks smooth, slowly pour in the chocolate. Give it one final mix using a rubber spatula to make sure all of the chocolate is incorporated.
Peanut Butter Filling
Add all of the ingredients except the icing sugar in a large bowl and mix using a hand held mixer. Mix until pale and creamy, 2-3 minutes.
Add the icing sugar a little at a time, beating well between each addition.
Kosher Salt: if you don't have kosher salt and only have fine salt, halve the amount of salt needed. 1 teaspoon kosher salt = ½ teaspoon fine saltButter: I use European style butter, like Kerry Gold, in my recipes. This makes it much creamier and richer.Scale: I highly recommend using a scale for your baking, it will be much more accurate. The peanut butter frosting is adapted from Serious Eats The cake is adapted from Ina Garten