Preheat the oven to 170°C (gas mark 3 or 325 °F).
Combine the sugar, butter and egg yolks in a bowl. Beat until pale. Whip the egg whites into soft peaks. Add the egg whites and flour to the mixture, one spoonful at a time, alternating between the two until they are used up. Add the finely grated lemon zest.
Grease the madeleine pans and dust lightly with flour. Place a small spoonful of mixture into each mould. Bake in the oven for 10 to 15 minutes, but do not leave the madeleines in for any longer or they will dry out.
Turn them out of the pan as soon as they come out of the oven and leave to cool.
Recipe courtesy of The Monet Cookbook: Recipes from Giverny by Florence Gentner, Francis Hammond (Photographer)