Put the butter in a bowl, then sift in half of the icing sugar. Beat well with a wooden spoon. Sift in the rest of the icing sugar and beat again until light and creamy. Mix in the grated orange and lemon zests, then beat in the liqueur, a few drops at a time, beating well after each addition.
Chill overnight so the flavours blend. Serve cold (you can keep this in the fridge for at least a week, or freeze for up to a month).