Heat oven to 180`c and line a 12-hole muffin tin with cases. In a large mixing bowl, mix the sugar, flours, baking powder, mixed spice and orange zest and put aside. Beat together the butter and brown sugar in a large bowl till light and pale. Beat in the eggs to incorporate. Fold in the grated carrots, followed by the flour mixture. Divide the mixture between cases and bake for 20-22 minutes until a skewer poked in comes out clean. Cool on a wire rack while you make the chocolate butter icing. Meanwhile, prepare the frosting by beating together the butter, icing sugar and grated orange zest. Finally, beat in the melted chocolate till well combined. Pipe the frosting onto the cooled cup cakes, decorate with the edible decorations and serve.