In a mixing basin, melt the chocolate gently over a pan of hot, but not boiling water. When it has melted, stir in the butter until it is very soft, but not quite melted. Add the orange zest.
Leave the melted chocolate and butter mix to cool (dip the basin briefly in cold water). As it cools but before it sets, whip the cream and the icing sugar together until thick, but still soft and loose, then fold quickly but thoroughly into the chocolate and butter.
Spread into a spring-form cake ring, sitting on a flat plate or cake board (on which you will serve the cake), levelling off the top with a spatula.
Refrigerate for at least three hours to set firm. Run a warm, damp cloth around the outside of the spring-form tin, then open the spring to release it and carefully lift it off the plate.
Decorate with shavings or gratings of chocolate and the crystallised zest. Keep chilled until serving, then cut into slices and slide a warmed cake slice underneath to release them.