Melt the butter in a small bowl and set it aside to cool.
3/4 cup (170g) unsalted butter
Sift the flour, baking powder, and salt into a medium bowl and set it aside as well.
1 cup (125g) all-purpose flour, 1/2 tsp baking powder, 1/4 tsp salt
Beat the eggs and sugar at medium speed for 2 minutes.
2/3 cup (133g) granulated sugar, 2 large eggs
Add in the vanilla and lemon zest and beat for another minute.
1 tsp vanilla extract, 2 tsp zest of 1 lemon
With the mixer on low speed, add in 1/4 of the flour mixture at a time, mixing in between. Scrape down the bowl as needed and mix until just combined.
Increase the speed to medium and slowly add the butter. Mix until the batter is smooth and thick.
Cover and chill the batter for a minimum of 30 minutes or overnight. (** see notes)
When ready to bake: Preheat the oven to 350F. Melt a tablespoon of butter and brush the shells of a madeleine pan with it and scoop a heaping tablespoon of batter into each shell.
1 Tbsp unsalted butter
Bake for 10-12 minutes until lightly golden on the edges and a toothpick inserted into the center comes out clean. Let the cookies cool in the pan for 5 minutes then carefully transfer them to a cooling rack.
Rinse the pan with cold water to cool. Wipe clean and dry. Repeat with the remaining batter.
Notes
Leftovers can be stored in an airtight container at room temperature for 3-5 days.*Room temperature eggs – It's really important to have your eggs at room temperature before you start beating them with the sugar in order to maximize the volume and reach the right consistency for the batter.**Chilling the dough – For best results, chill the dough for just 30 minutes. This is the perfect amount of time for the flour to relax without having the butter solidify again.