In a large bowl mix the flour with the baking powder. And set it aside
In a separate bowl beat together the eggs and the sugar for about 5min or until they triple in volume. Beat in the vanilla extract and the lemon zest. With a rubber spatula, fold in the flour mixture and the poppy seeds, then fold in the butter.
Cover the batter with plastic wrap, making sure the wrap touches the surface of the batter to create an airtight seal. Chill in the refrigerator for at least 3 hours. ( You can keep the batter chilled in the refrigerator for up to 2 days).
When you’re ready to bake the madeleines, preheat the oven to 180C degrees.
Grease the madeleine pan with cooking oil.
Bake for about 10 to 12 minutes. The madeleines are done when the tops spring back after lightly pressed with your finger. Invert the pan onto the counter. Transfer the warm madeleines to a wire rack to cool.
Dust with confectioners’ sugar, if desired, before serving. Madeleines are best enjoyed right after baking. However, they would keep well in the refrigerator in an airtight container for up to 2 days. (*Check my make-ahead tips in the post above)