Preheat the oven to 180 C / Gas mark 4. Lightly butter a baking tray.

Cream margarine and sugar together until light and fluffy; stir in flour and zest until you have a dough. Then roll out the dough on a floured side until its about 2 inch thick.

Cut the shortbread using a cutter or just cut into fingers then place onto a baking tray. Sprinkle with caster sugar and chill in the fridge for 20 minutes.

Then bake in the preheated oven until golden brown, once cooked, take out the oven and transfer the shortbread to a cooling rack.