Brush a tray of madeleine moulds with melted butter and coat with plain flour, then tap the tray to remove the excess flour.
Whisk the eggs and sugar in a bowl until thick and pale and the whisk leaves a trail when lifted. Sift the flour, then gently fold into the egg mixture. Fold in the melted butter and lemon and orange rinds. Spoon into the moulds, leaving a little room for rising.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 12 minutes or until golden and springy to the touch. Remove the madeleines from the tray and leave to cool on a wire rack.