In a small bowl, sift together the flour and 55g of caster sugar. Beat the egg whites using an electric whisk until foamy, then add the cream of tartar, a pinch of salt and the lemon juice. Turn the speed up to high and continue to whip the egg whites until they just begin to hold soft peaks. Gradually add the remaining sugar, a spoonful at a time. Be careful not to overwhip – the peaks should be soft and cloud-like. Using a spatula, gently fold the flour and sugar mixture into the egg white mixture, ensuring that everything is well incorporated.