Place oven racks in upper and lower thirds of oven and preheat to 350°F (175°C).
Brush molds with melted butter.
Sift together flour and baking powder into large bowl, then whisk in lemon zest.
Scrape seeds from vanilla bean with a tip of a paring knife, into a smaller bowl (save pod for another use) then add sugar and rub together until vanilla is well dispersed.
Whisk in eggs, milk, honey and melted butter, then fold into flour mixture, until just combined.
Place a rounded tablespoon of batter into each mold, filling about ⅔ full.
Bake on bottom rack for about 5 minutes, then rotate tray 180° and move to top rack for another 5 to 6 minutes, or until golden brown around the edges.
Remove from the oven and take pastries from the tray. TIP: if the Madeleines don't come out easily, use a butter knife or small wooden tool to help remove them from the molds. (If you re-use the molds right away, make sure to wash them well, and brush with butter again.)
Place on cooling rack and serve as is, slightly warm, or add a light dusting of powdered sugar when cool.