Cake: Preheat oven to 150 degrees C. Grease 20cm round cake tin with butter and dust with cocoa powder.
Melt the chocolate and butter together in the top of a double boiler or carefully in a microwave. Remove from heat and stir in sugar, cocoa, eggs and vanilla.
Pour into the prepared cake tin and bake for 30 minutes. Cool in the tin for at least 10 minutes and then turn out onto a wire rack to cool completely.
Ganache: Melt the cream and chocolate together in the top of a double boiler or carefully in the microwave, stirring until smooth and shiny.
To Serve: Once the cake is completely cooled pour the ganache over the top of the cake and either serve warm as is or wait until the ganache hardens.