Preheat oven to 350 degrees F (175 C). Butter two 6-inch round cake pans and line with a round of parchment paper on the bottom.

In a medium bowl, whisk together the flour, sugar, and salt.

Boil the water. Meanwhile, melt the butter in a small saucepan. Stir in the cocoa powder, then the boiling water; boil for 30 seconds and remove from heat. Pour over the dry ingredients and give it a gentle stir.

Whisk together the buttermilk, egg, baking soda and vanilla. Stir into the chocolate batter and mix just until combined.

Pour into the prepared pans and bake until a toothpick comes out nearly clean, about 20 minutes (check at the 15 minute mark). Remove from oven and allow to cool in the pans for 10 minutes before turning out onto a cooling rack.

Place the cream cheese in the bowl of a stand mixer fitted with the whisk attachment. Beat until smooth. Add the peanut butter and beat again, scraping down sides of bowl as necessary. Add the butter and beat until smooth again. Beat in the powdered sugar and vanilla.

Once the cakes have cooled completely, cut each one in half horizontally. You can either leave the dome on one of the cakes or level them both off.

Place one layer on a cake stand or plate and put strips of parchment just underneath to keep the surface tidy. Spread with frosting to create a layer that's about 1/4-inch thick. Repeat with remaining layers, ending with the domed layer if using.

Frost the top of the cake and put a thin "crumb coating" on the sides. Refrigerate cake until the crumb coat is firm, then add the rest of the frosting to the sides (or save a bit for eating on its own). Remove the parchment strips, slice and enjoy!