Preheat the oven to 190°C/375°F/gas 5. Grease a flat baking tray, then line with a sheet of baking paper.
Beat the butter, then grate in the orange, lemon and lime zests. Cream the butter and zests with the icing sugar until light and fluffy.
Fold in the flour and baking powder, then use your hands to form a dough – do not overmix.
For rectangular shortbread, roll out to about 1cm thick and cut into fingers. For round biscuits, roll into 20 even-sized balls. Prick the surface of each. Reroll any scraps and repeat.
Put your shortbread onto the tray, then chill in the fridge for 10 to 15 minutes, before baking in the oven for 12 to 15 minutes until pale and crisp.
Remove from the oven and allow to cool for 5 minutes, then transfer to a cooling rack and sprinkle with sugar. This shortbread will keep for up to a week in an airtight container.
