Beat the butter and sugar with an electric whisk until soft and light. Add the lemon rind, then rub in the ground almonds and gram flour with your fingers, working as lightly as you can. Pat the mixture out, approximately 20cm thick, in the bottom of a tin and bake in a moderate oven (160ºC/325ºF/Gas 3) for 15 minutes. Remove and cut into biscuit shapes with a sharp knife then return to the oven for another 5 minutes. Cool slightly then cut along the score marks. Using a spatula, remove carefully to a rack to cool completely.