Preheat the oven to 180C/350F/gas mark 4
Use a 20cm/8 inch round cake tin.
Grease and line the tin. Cream the butter and the sugar until light. Whisk in the sour cream, mixed spice, orange rind, juice and coffee. Mix well. Toss the carrots and walnuts in the flour and gradually stir into the beaten mixture. Pour the mixture into the tin and bake for 1 1/2 hours. Cool in the tin for half an hour. Remove and finish cooling on a wire rack.
Topping: Cream the cheese and butter together. Slowly sift in the icing sugar and continue beating until the mixture is quite smooth. Stir in the vanilla and lemon juice. Spread 2/3 of the mixture on top of the carrot cake. Place the remainder in a piping bag and pipe rosettes around the cake.
