Chocolate Madeleines (madeleines au chocolat)

Instructions

1. Preheat the oven to 375°F. To a small bowl, add the butter and melt in the microwave until completely liquid. Set aside to cool. 
2. To a medium bowl, sift and add the flour, baking cocoa, salt and baking powder.  Stir everything together to combine. Set aside.
3. In a large bowl, or the bowl of a stand mixer, beat the eggs and sugar together until thick and pale in color (about 1 minute). Add the vanilla extract and briefly mix to combine. 
4. You're going to want to add the dry ingredients and melted butter in alternating batches now. First add 1/3 of the dry ingredients to the batter, mix, then add half of the melted butter. Mix again. Add another 1/3 of the dry ingredients, mix, then add the remaining butter. Mix and add the remaining flour; give the batter 1 final mix. Stop to scrape the bottom of your mixing bowl with a rubber spatula to make sure everything is incorporated well. Don't over mix the batter; just give it a brief mix until everything is combined. 
5. Place your batter into your freezer for a few minutes. While the batter is chilling, heat 1 tablespoon of butter until it's just softened but not melted (about 5-7 seconds in the microwave). Brush the softened butter into your madeleine molds using a pastry brush. Use a sieve to dust flour into the buttered molds. Tap the pan upside down to get rid of any excess flour. 
6. Remove the batter from the freezer. The batter should be a little stiff - similar to a light mousse (see note). Use a tablespoon measuring spoon to scoop out a heaping spoonful of batter into each mold (you will want to use another spoon to help you actually scoop the batter out and into the mold). You want to fill each mold about 2/3 full; do not completely fill the molds as the madeleines will expand when they bake. 
7. Bake the madeleines for about 12 minutes, just until the madeleines spring back with a gentle press of the finger. Do not open the oven door before the 10 minute mark to check on your madeleines - this could cause their little signature "humps" to fall. Let the madeleines rest in the pan for a minute before gently nudging the madeleines out (using a knife) onto a wire cooling rack.
8. Once the madeleines are cool, drizzle white chocolate over each madeleines, then garnish with sprinkles. (Alternatively, you can just dust them with powdered sugar.)