Melt the butter in a saucepan over low heat or in the microwave. Set aside.
Sift together the flour and baking powder. Set aside.
In a large bowl, whisk together the eggs and sugar until light and fluffy.
Gradually whisk in the dry ingredients until just combined.
Whisk in the melted butter.
Grease the madeleine molds with butter or nonstick cooking spray, if not using silicone mold.
Transfer the madeleine batter to the piping bag, filling it about ¾ full. Gently squeeze the piping bag to deposit the batter into the molds. Fill each mold about ¾ full.
Refrigerate for at least 30 minutes.
Preheat the oven to 428°F.
Place the madeleines in the preheated oven and immediately reduce the temperature to 400°F. .Bake for 10-15 minutes, until the madeleines are golden brown and springy to the touch.
Let the madeleines cool in the molds for a few minutes before inverting them onto a wire rack to cool completely.
NOTES:
Madeleines are best served fresh, but they can also be stored in an airtight container at room temperature for up to 2 days.
For a more intense flavor; you can add a zest of a lemon or a teaspoon vanilla extract.