To begin, place a small saucepan over medium heat and melt the unsalted butter. Once melted, allow it to cool slightly.

In a medium bowl, whisk together the flour, baking powder, and salt.

In a large mixing bowl, beat the eggs and granulated sugar together using a hand mixer or stand mixer. Beat on high speed for about 5 minutes or until the mixture is thick and pale and forms ribbons when the beaters are lifted. This step incorporates air into the batter, which helps create the light and fluffy texture of the Madeleines.

Reduce the mixer speed to low and add the vanilla extract and lemon zest if using. The lemon zest adds a subtle citrusy aroma that complements the buttery richness of the Madeleines.

Gently fold the flour mixture into the egg mixture using a spatula.

Slowly pour the melted butter into the batter, folding it in gently until thoroughly combined. The butter should be cool enough that it doesn't deflate the batter.

Cover the bowl with plastic wrap and refrigerate for at least 1 hour, preferably overnight. 

While the batter chills, prepare your Madeleine pan. Grease the pan generously with butter or non-stick spray and dust with a light coating of flour.

Preheat the oven to 375°F (190° C). Place a rack in the centre of the oven.

Spoon the cold batter into the prepared Madeleine pan, filling each mould three-quarters full. Be cautious not to overfill the Madeleines, since they will puff up when baking.

Bake the Madeleines for 10-12 minutes, or until the edges turn golden brown and the centres spring back when softly pushed. The distinctive "hump" should form in the centre of each Madeleine

Remove the pan from the oven and allow the Madeleines to cool in the pan for a few minutes. Then, gently remove them from the moulds and transfer them to a wire rack to cool completely. If desired, dust the Madeleines with powdered sugar before serving.