Sift the flour, sugar, cocoa powder, bicarbonate of soda, baking powder and salt into a large bowl and whisk together.
In a separate smaller bowl mix together buttermilk, eggs and vanilla. Add this to the dry ingredients and beat to make a smooth batter, then add the coffee, whisking as you go.
Separate between two lined 23cm springform cake tins and bake for 40/50 minutes, until a skewer comes out clean. Leave to cool for 10 minutes then carefully remove from their tins and leave to cool fully on a wire rack.
Meanwhile, heat the water in a large pan and add the marshmallows. Stir until they melt into a smooth mixture.
Whisk together the cream cheese and icing sugar. Set aside and whisk the thick cream in a separate bowl to make stiff peaks.
Fold the whipped cream into the marshmallow mixture and spread over one of the cakes - it helps if it is placed back in a springform tin with lined sides to get cleaner edges at this point!
Place the other cake on top and leave to set in the fridge.
Place the chocolate in a bowl and heat in 20 second bursts in the microwave until completely melted, then stir in the coconut oil.
Place the whole thing on a wire rack over a tray (to catch drips) and pour the chocolate mixture over the whole cake. Leave to set again before serving.