Preparation
Melt and Cool the Butter: Gently melt the butter and set it aside to cool to room temperature.
Prepare the Pan: Brush your madeleine tray generously with a little melted butter, then lightly dust it with flour. Chill the prepared tray in the refrigerator.
Combine Dry Ingredients: Sift the flour and baking powder together into a bowl. Add the salt and lemon zest, then mix briefly and set aside.
Mixing the Batter
Whisk Eggs and Sugar: In a separate mixing bowl, whisk the eggs and sugar until the sugar is fully dissolved and the mixture is pale and ribbon-like (this means it forms a trail when the whisk is lifted).
Fold in Dry Ingredients: Gently fold the dry ingredients into the egg mixture until they are just combined. Be careful not to overmix.
Incorporate Butter and Flavoring: Add the cooled melted butter and vanilla essence to the batter. Mix gently until just combined.
Chilling and Baking
Chill the Batter: Cover the bowl and refrigerate the batter for at least 30 minutes. For the best results and the signature 'hump,' you can chill the batter overnight. If you are in a rush, you can finish by chilling the batter in the freezer for 10 minutes.
Fill the Molds: Transfer the batter to a piping bag (or use a small spoon) and fill each mold in the chilled tray to about two-thirds full.
Bake: Bake the madeleines in a preheated oven at 210∘C (410∘F) for 8 minutes.
Then, without opening the oven door, reduce the heat to 180∘C (356 ∘ F) and bake for another 5 minutes, or until the edges are golden brown and the cakes spring back lightly when touched.