Chocolate Orange Truffle Cake

500g/1lb20z top quality dark cooking chocolate
100g/31/2oz very soft unsalted butter
the zest of an orange
100g/31/2oz icing sugar
500ml/18floz whipping cream

To decorate (optional):
shavings of plain chocolate
crystallised orange zest (thin strips of blanched zest, dipped in beaten egg white then in caster sugar and left in a cool dry place to harden)

Method:
In a mixing basin, melt the chocolate gently over a pan of hot, but not boiling, water. When it is liquid, stir in the butter until it is very soft, but not quite melted. Add the orange zest. Leave the melted chocolate/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.