Sift the flour with the baking powder. Whisk the eggs and sugar until foamy. Beat in the lemon zest. Fold in the flour in three batches, adding the butter with the last batch. Chill for 20-30 minutes until the dough stiffens. Heat the oven to 450F. Grease and flour several madeleine molds. With a large spoon pour the dough into the molds so that they are two-thirds full. Bake the madeleines for 5 minutes. Reduce heat to 400F and continue baking until golden-brown, 5-7 minutes. Transfer the madeleines to a wire rack to cool.
