Bring eggs to room temperature. Beat eggs and sugar in a mixer. When mixed, add the vanilla, lemon zest, flour, and baking powder. Mix again. Then, add the buttermilk and melted butter. Place the bowl in the refrigerator for an hour. Scoop the batter out with a cookie scoop and place in each cavity. Bake in a 375 degree oven for 10 minutes or until slightly browned. Slightly pop out the Madeleines with a fork and place on a cooling rack. Make a glaze of the powdered sugar and lemon juice. Brush it on the shell side of the Madeleine. Serve while warm.