Warm the butter so that it is just liquid but not at all coloured, and leave to cool a little while you beat the whites and yolks of the eggs separately. The beaten whites should just hold a peak. Fold in the yolks, sugar and grated lemon zest, beat in the butter and stir in the flour. Beat well and turn into well-greased bun tins or patty pans. Bake at 400°F (200°C, Gas Mark 6) for 10 to 15 minutes.