After an hour or two, take out the cake and apply the rest of the buttercream to the cake. To prepare the chocolate ganache, chop the chocolate into even pieces and set aside. Heat the double cream in a small saucepan. Once you see tiny bubbles, take it off the heat immediately and stir in your chopped chocolate. You should have a glossy ganache. If the chocolate doesn't melt in the cream, heat it on a very low heat and stir until glossy.