Sift the flour and baking powder into the processor bowl.

Add the coffee essence, margarine, eggs and caster sugar and process for 20 seconds. After 10 seconds stop the machine and scrape down the ingredients.

Pour into a greased 23cm (9 inch) square cake tin and bake at 180°C, 350°F, Gas No. 4 for 40 - 45 minutes.

Place the nuts into the food processor, chop finely and reserve.

Place the butter, icing sugar and orange rind together into the processor bowl and mix together well.

Split cake in half and sandwich together with two thirds of the butter icing. Ice cake all over with remaining butter icing and coat sides in chopped nuts. Pipe the rest of the icing on the cake and decorate with the mandarins.