Prepare the Batter:

In a medium bowl, whisk together the flour and baking powder; set aside.

In a large mixing bowl, beat the eggs and granulated sugar using an electric mixer on high speed until the mixture becomes pale and thick, approximately 3-4 minutes.

Add the vanilla extract and lemon zest (if using) to the egg mixture, mixing until combined.

Gently fold in the flour mixture until just incorporated.

Combine the melted butter and heavy cream in a separate bowl. Gradually fold this mixture into the batter until smooth.

Chill the Batter:

Cover the batter with plastic wrap and refrigerate for at least 1 hour, or until it firms up slightly. This step is crucial for achieving the characteristic "hump" on the madeleines.

Prepare the Madeleine Pan:

Preheat your oven to 375°F (190°C).

Generously grease a madeleine pan with butter and lightly dust with flour, or use a non-stick baking spray to ensure easy release.

Bake the Madeleines:

Spoon the chilled batter into the prepared madeleine pan, filling each mold about three-quarters full.

Bake in the preheated oven for 10-12 minutes, or until the edges are golden brown and the centers spring back when lightly touched.

Remove the pan from the oven and immediately tap it on a countertop to release the madeleines. Transfer them to a wire rack to cool completely.

Serve:

Once cooled, dust the madeleines with powdered sugar before serving.