Prepare the batter. In a small saucepan melt butter taking care not to burn it. Remove the saucepan from the heat pour the butter in a cup or small bowl to cool it down quickly. Sift together the flour, salt and baking powder and set aside.
In another bowl, whip the eggs and sugar until thick and pale for about 5 minutes. Gradually add the milk while mixing. Fold in the sifted flour and the lemon zest until well incorporated. Finally whisk in the butter trying to add volume to the batter. Cover the bowl with plastic wrap, and chill in the refrigerator overnight.
The next day
On the day of baking, preheat the oven to 400°F. While the oven is preheating, freeze the madeleine mold for 15 minutes. Take the mold out of the freezer and brush it generously with melted butter. Sprinkle with flour if you wish. Tap off any excess flour.
Fill a pastry bag fitted with a plain tip with the batter and pipe it into the Madeleine mold, alternatively, you can spoon the batter into the mold. Bake in the preheated oven for 5 minutes until they begin to color. Lower the oven temperature to 325°F and continue cooking for 5-7 minutes. Remove the Madeleines from the oven, unmold immediately, and leave to cool on a wire rack.
Recipe by Fraiche Pantry at https://www.fraichepantry.com/2014/03/21/madeleines/