Preheat oven to 375°F (190°C). Grease and lightly flour a madeleine pan.

In a mixing bowl, whisk together the sugar and eggs until pale and slightly thickened.

Add vanilla extract and, if using, lemon zest; whisk to combine.

In another bowl, sift together the flour, baking powder, and salt.

Gradually fold the dry ingredients into the egg mixture until just combined.

Gently fold in the melted, cooled butter until the batter is smooth.

Cover and refrigerate the batter for at least 30 minutes (this helps form the signature hump).

Spoon the batter into the prepared madeleine pan, filling each mold about three-quarters full.

Bake for 10–12 minutes, or until the edges are golden and the centers spring back when lightly pressed.

Immediately remove from the pan and cool on a wire rack.

Dust with powdered sugar before serving.