Preheat oven to 350 degrees Fahrenheit.Melt butter & sugar in a bowl set over simmering water. Add in chocolate, stirring well.Add the egg yolks to the chocolate mixture, then mix in the all the nuts, orange rind & cinnamon. Whisk yolks until stiff, but not dry. FOLD in egg whites to the chocolate mixture. (You don’t want to knock the air out by mixing too hard.)Pour into a buttered 8in spring form cake pan.Bake for 45 minutesLeave the cake in the pan to cool for about 20 minutes. It is a fragile cake, so handle with care.The top will deflate & crack a little as it cools. Dust with powdered (icing) sugar.