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.
Melt the dark chocolate, either in a bowl over a pan of gently simmering water or in short bursts in the microwave, set aside to cool slightly.
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.
Fold in one third of the flour mixture, followed by half of the buttermilk mix. Fold in another third of the flour, then the remaining buttermilk, then the rest of the flour.
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.
Leave the cakes to cool in the tins for 10 minutes then carefully turn them out onto a wire rack, peel off the baking parchment and leave to cool completely.
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.
Set aside until the ganache has cooled and become thick and spreadable, stirring it every now and again. This can take a while so don't leave it till the last minute! You can pop the bowl in the fridge to speed it up if you like but make sure that you keep a close eye on it and stir it regularly.
To assemble the cake, place one cake layer on a cake stand or serving plate and spread over one third of the ganache. Top with another cake layer and another third of ganache then repeat for the final layer and serve!