Cream the butter (margarine will do) and sugar until light and fluffy and add the lemon rind. Sieve the cornflour and baking powder together. Break the egg into a bowl and beat with a fork. Add a small amount (about a tablespoon) of egg and a similar amount of cornflour alternately to the creamed butter and sugar, beating well between each addition. Place about a dozen paper cake cases in a baking tin and add a teaspoonful of mixture to each case (use the back of another spoon to push it off). Place in a pre-heated oven at 375F/180C/Gas mark 4 and bake 10 minutes until firm and golden (don't let them get beyond a golden colour). Allow the cakes to cool on a rack and then add a dusting of the icing sugar.