Melt the butter over low heat, then set it aside until warm but not hot. Whisk the flour, baking powder, and salt together in a separate bowl.

Whisk the room-temperature eggs and sugar for 3 to 5 minutes, until pale, slightly thickened, and fluffy. Whisk in the vanilla and optional lemon zest.

Fold the dry ingredients into the egg mixture just until combined, then fold in the warm melted butter. Cover and refrigerate the batter for at least 30 minutes, or up to 1 hour for a better hump and sharper shell pattern.

Heat the oven to 375°F (190°C). Generously butter a madeleine pan, dust it lightly with flour, and tap out the excess.

Spoon or pipe the chilled batter into the molds, filling each about two-thirds full. Do not spread the batter completely to the edges.

Bake for 8 to 11 minutes, until the edges are golden brown and the centers spring back lightly when touched. Cool in the pan for about 5 minutes, then transfer the madeleines to a rack to cool completely.

Melt the chopped chocolate gently in a microwave using 15- to 20-second intervals and stirring between each interval, or melt it over a double boiler. Stir in the optional oil for a smoother coating.

Dip the scalloped or flat side of each completely cool madeleine into the chocolate, or spread chocolate over one side. Place chocolate-side up on parchment and let the coating set at room temperature or briefly in the refrigerator.