Beat the butter and sugar until pale and fluffy, then beat in your chosen citrus zest, followed by the egg. Sift the flour and salt into a bowl. Using a metal spoon, quickly fold the flour into the butter mixture followed by a tablespoon of your chosen citrus juice. Spoon the mixture into the paper cases and bake for 15 minutes until they are well-risen and golden. If you press a cake lightly, the sponge should bounce back. Cool on a wire rack.