Heat oven to 180C/160C fan/gas 4 and line a 12-hole muffin tin with cases. In a large mixing bowl, mix the sugar, flours, bicarbonate of soda, mixed spice and orange zest. Whisk together the eggs and oil, then stir into the dry ingredients with the grated carrot. Divide the mixture between cases and bake for 20-22 mins until a skewer poked in comes out clean. Cool on a wire rack before icing.
For the icing, beat the butter until really soft, then beat in the soft cheese, icing sugar and vanilla. Use a palette or cutlery knife to swirl the icing on top of the cakes, then sprinkle with decorations.