Preheat your oven to 375°F (190°C) to ensure even baking and a perfect golden color.
Generously brush the madeleine molds with melted butter to prevent sticking and create a golden crust.
In a mixing bowl, whisk together 3 large eggs and 2/3 cup granulated sugar until light and frothy, about 3-5 minutes.
Sift together 1 1/4 cup flour, 1 teaspoon baking powder (if using), and rounded 1/8 teaspoon salt in another bowl. Gradually fold into the egg mixture with a rubber spatula, being careful not to deflate the batter.
Gently fold in the zest of one small lemon and 9 tablespoons melted butter until smooth and well combined.
Spoon batter into the prepared madeleine molds, filling each about 3/4 full to allow room for rising.
Bake the madeleines for 10-12 minutes until edges are golden and centers spring back when lightly touched.
Remove the pan from the oven, cool for a few minutes, then gently tap to release madeleines onto a cooling rack.
In a small bowl, whisk together 3/4 cup powdered sugar, 1 tablespoon fresh lemon juice, and 2 tablespoons water until smooth. Adjust consistency if needed.
Once madeleines are completely cool, dip the tops into the lemon glaze and place back on the cooling rack to set.