6 tablespoons (85 grams) unsalted butter, melted and cooled
Powdered sugar, for dusting (optional)
In a small bowl, whisk together the flour, hot chocolate mix, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the whisk attachment (or in a large bowl using a hand mixer fitted with whisk beaters), beat the eggs on low speed for 1 minute, then increase the speed to medium-high and gradually beat in the sugar. Continue beating for 2 to 3 minutes until soft peaks form. Add the vanilla and beat on low speed until combined. Using a rubber spatula, fold in the flour mixture until just combined, then fold in the melted butter until just combined. Cover with plastic wrap, placing the plastic wrap directly on the surface of the batter. Place the bowl in the refrigerator to rest for 3 hours, or up to overnight.
Preheat the oven to 375°F (190°C). If you have a nonstick madeleine pan, lightly butter the pan. If you have a madeleine pan that is tin or made from another metal, butter and flour the pan. If you have a silicone madeleine mold, you don't need to butter or flour the mold. (If you have two pans, prep both.)
Remove the batter from the refrigerator. Spoon a heaping teaspoon of batter into each well of the madeleine pan molds, filling them almost to the top.
Bake for 9 to 11 minutes, or until the madeleines are lightly puffed in the middle and the tops spring back after a light tap. Remove the madeleines from the oven. As soon as the madeleines are cool enough to touch, remove them from the pan and place them on a wire rack. (If after 2 minutes the cookies are still too hot, use a butter knife to gently pop them out.) Let the pan cool, then prep the pan again and bake off the remaining batter.
When completely cool, lightly dust each cookie with confectioners' sugar (if desired). Store in an airtight container at room temperature for up to 4 days; or in an airtight zip-top bag in the freezer for up to 2 months.
Reprinted with permission from Guittard by Amy Guittard, copyright (c) 2015. Published by Chronicle Books.