In a small bowl, combine the flour, baking powder, and salt. In a separate small bowl, combine the sugar, lemon zest, and vanilla beans. Use your fingers to rub the zest and beans into the sugar, releasing their aromatic oils.

In the bowl of a stand mixer fitted with a paddle attachment, beat the sugar mixture with the eggs on medium high speed until pale and thick, about 3 minutes. Add the vanilla extract. Use a spatula to fold in the melted butter, and then the flour.

Cover and refrigerate batter for 3 hours.

Fill the shells of the madeleine pan with the chilled batter almost full. Bake until centers are puffed and edges are golden brown, 10-12 minutes. Let cool for a few minutes in the pan, then pop out onto a wire rack to cool completely.

Madeleines are best the day they are made, but may be stored for up to 2 days in an airtight container at room temperature.