Preheat the oven to 180 degrees Celsius.
Place the chocolate and butter in a heatproof bowl, over a small saucepan, half filled with water. Make sure the water doesn't touch the base of the saucepan, or you risk overcooking your chocolate. Nobody likes overcooked chocolate.
Once they're melted, sieve the cocoa and sugar, concurrently, into the melted chocolate. I'm normally too lazy to sieve, but I learnt from Ottolenghi that sieving cocoa together with sugar discourages the cocoa from clumping up, which to be honest is my pet peeve with cocoa. A revolutionary tidbit from a revolutionary man.
Mix the sugar and cocoa into the chocolate, and continue to do so until most of the sugar grains have dissolved. Add the coffee milk mixture and the almond milk, and continue mixing until combined. Take the saucepan off the heat, and very carefully remove the bowl from the saucepan of water. Use a tea towel and stand back so you don't steam burn your hands or face. Been there done that.
Mix in the eggs, baking powder and vinegar, and once combined, pour into a greased springform tin. I used a 23cm tin, but I've also used 2 mini tins, to create a mini cake tower, which is fun. Whatever you do, don't go too much bigger than a 23cm tin, or you'll have a pancake, not a cake.
Place in the oven for 20 minutes, or until the cake is cooked through (note that this will depend hugely on what size cake tin you use, so watch it carefully)
While the cake is cooking, cream together the icing sugar and butter in a medium bowl. Continue beating until, as the name 'creaming' suggests, they become a light. creamy colour, with an airy-ish texture. Add the peanut butter and cocoa, and continue beating (be aware that cocoa might fly up as you start, I use my hand to cover the bowl and minimise the losses) until you have a buttercream. Once done, place it in the fridge until you're ready to ice
Once the cake has cooked, allow it to cool slightly before running a knife around the edge, and removing it from the tin. Allow it to cool completely before icing, or the buttercream will melt off. Delicious, but not visually grabbing. I like to top mine with berries and a pinch of sea salt.