Put the cream into a small saucepan and bring to the boil. Remove from the heat and add the chocolate. Stir until all the chocolate has melted and you have a smooth mixture. If necessary, cool until thick enough to spread, then spread the mixture over the cake. Allow the ganache to set.
Chocolate Butter Cream
Put the icing sugar, cocoa, butter and vanilla in a mixing bowl.
Add warm water, little by little, beating well so you have a spreadable paste. Spread all over the cake and allow to set.