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Heat oven to 180C/160C fan/gas 4. Grease and line the bases of 2 x 20cm round cake tins with baking parchment. Beat the butter, sugar, eggs, flour, bicarb, zest and the mixed spice together, then stir in the carrot and courgette. Divide the mixture between the tins and bake for 20-25 mins or until a skewer inserted in comes out clean. Cool.
To make the topping, mix enough of the orange juice into the icing sugar to give a thick, but drizzly icing. Drizzle over the cakes, then scatter with the zest and leave to set.