In this recipe, which we based on one in Larousse gastronomique, baking soda is added as a leavening agent in order to accentuate the madeleine's telltale hump. (Some recipes state that letting the batter rest before baking creates a higher hump; we found the difference to be negligible.) Heat oven to 375°. Grease two 12-mold madeleine pans containing 3"-long molds with 2 tsp. butter. Melt 6 tbsp. plus 1 tsp. butter in a pot over medium-low heat; let cool for 10 minutes. Whisk together 1 3⁄4 cups cake flour, 1 cup sugar, 1⁄2 tsp. baking soda, finely grated zest of 2 lemons, and a pinch of salt in a bowl; fold in 4 beaten eggs until smooth. Add butter; fold until just mixed. Divide batter between molds, filling each about three-fourths of the way. Bake, rotating once, until golden brown, 15–20 minutes. Remove cookies from molds; let cool slightly. Makes 2 dozen.