Melt the chocolate gently in a basin over a pan of hot water.
Stir in the soft butter and orange zest until well combined.
Cool the mixture briefly in cold water without letting it set.
Whip the cream and icing sugar together until thick but soft.
Fold the whipped cream thoroughly into the chocolate mixture.
Spread the mix into a spring-form ring on a flat plate and level the top.
Refrigerate for at least three hours until firm.
Remove the ring by running a warm damp cloth around the edge.
Decorate with chocolate shavings and crystallised zest before serving chilled.