Preheat the oven to 180C/350F/gas mark 4. Grease three 18-20cm/7-8in round cake tins and line the bases with baking parchment.
Whisk together the cocoa powder and hot coffee until smooth then whisk in the buttermilk, set aside. Sift together the flour, bicarbonate of soda and salt, set aside.
Place the butter, sugar and vanilla extract in a large bowl and whisk with an electric mixer until smooth and light. Whisk in the eggs one at a time, whisking really well after each addition, then whisk in the melted chocolate.
Divide the batter evenly between the tins (I use a scale to get them exactly even) and level the surface. Bake for 25-30 minutes until a skewer inserted into the center comes out clean.
To make the ganache, place the dark and milk chocolates into a large bowl. Pour the cream into a saucepan and heat until it is just boiling. Pour the cream over the chocolate and leave to sit for a couple of minutes then stir until the chocolate is melted and smooth.