Preheat the oven to 190°c (170 fan/ 375F/ Gas 5) and line a large baking tray with baking paper.
Cream together the butter and sugar until light and fluffy.
Beat in the egg, lemon zest (from 2 lemons) and the lemon juice from just 1 of the lemons.
Stir in the flour and baking powder and mix to form a dough. If the mixture is a little dry add more lemon juice (from the second lemon) and mix and continue to do this until a smooth dough forms. Use your hands to bring the dough together in the bowl.
Cover the bowl with clingfilm and chill the dough in the fridge for 15 minutes.
Take tablespoons of the dough and roll into balls. Place on the baking sheet (you should get about 20 cookies). Flatten down slightly with the back of a spoon.
Bake in the oven for 15 minutes. Take out of the oven and flatten down again with the back of a spoon. These biscuits do not spread and are meant to be thick, round and bitesize.
Once cool sift over the icing sugar and serve!