Cut cake in half horizontally, smear with jam and reassemble. Beat butter, icing sugar, cocoa, vanilla and juice in a mixer on high speed for 4 minutes or until light and creamy.
Ice cake with the butter cream and refrigerate for 5 minutes. Cut an 8cm hole in the cake's centre using a long sharp knife; fill with snakes and coins.
Line the inside of a mixing bowl just larger than the cake with aluminium foil, then freeze for 10 minutes. Smear melted dark chocolate over the foil until set. Carefully invert bowl and remove chocolate-lined foil. Slowly peel off foil and put chocolate shell over cake. Refrigerate for 10 minutes.
Decorate chocolate shell with Smarties, using melted milk chocolate to stick them on. Write 'Happy Birthday' in white chocolate, on top of cake. Serve with a wooden mallet.