1. The night before, prepare the chocolate Madeleine batter: in a saucepan cook the butter until it is golden – this is known as ‘hazelnut butter’. Remove the saucepan from the heat and strain the butter to cool it down quickly. In another bowl whip the eggs and sugar until thick and pale, gradually add the milk. Add the sifted flour, cocoa powder, and baking powder and cut and fold until well incorporated. Finally add the butter with a whip, trying to add volume to the batter. Cover the bowl with plastic wrap, and chill in the refrigerator overnight.
2. Also the night before, prepare the lemon Madeleine batter: follow the same procedure as for the chocolate Madeleine batter, replacing the cocoa powder with the finely grated lemon zest.
3. On the day of recipe preparation, preheat the oven to 200°C (400°F). Brush the Madeleine baking molds with melted butter and sprinkle with flour. Tap off any excess flour.
4. Fill a pastry bag fitted with a plain tip with the two batters and pipe the batter into the Madeleine molds, or spoon the batter into the molds. Cook them in the preheated oven for 5 minutes until they begin to color. Lower the oven temperature to 180°C (350°F) and continue cooking for 5-7 minutes. Remove the Madeleines from the oven, unmold immediately, and leave to cool on a wire rack.