This is yet another type of cookie I remember seeing regularly around the household come winter. Mom was of the "if it's not chocolate, it's not worth eating" philosophy, so this cookie is, in essence, dedicated to her. Cobbled together Frankenstein-style from a few of our family "crinkle" recipes, it is the most chocolaty cookie I could create that is not just a slab of chocolate. Slightly firm on the outside. Melt-in-your mouth goodness on the inside.
In a medium bowl, sift together the flour, cocoa powder, baking powder, and salt. Set aside.
Place the chocolate and butter in a large heatproof bowl set over a pan of simmering water, and stir occasionally until they are almost completely melted and combined. Stir in the espresso powder. Remove from the heat and continue stirring until the last remaining chunks of chocolate and butter are melted. Let cool to room temperature.
In the bowl of a standing mixer fitted with the paddle attachment, beat the eggs, granulated sugar, and brown sugar together on medium speed until smooth and the color of a very light cappuccino, 3 to 4 minutes. Add the vanilla and beat again until incorporated. Scrape the chocolate mixture into the bowl and beat until combined. Scrape down the sides and bottom of the bowl, and add the flour mixture all at once. On the lowest speed possible, beat the mixture until just incorporated; do not overmix. Cover the bowl tightly and refrigerate the dough for at least 2 and up to 24 hours.
Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Place the confectioners' sugar in a small wide bowl.
Remove the dough from the refrigerator. Scoop and form the dough into balls about 1 inch (2.5 cm) in diameter. Roll them in the confectioners' sugar to coat and place on the prepared baking sheets about 1. inches (4 cm) apart. If the cookies do not pick up enough confectioners' sugar at first pass (which is important for visual flair), go back and give the cookies a second roll.
Bake, rotating the sheets halfway through the baking time, until the cookies start to firm up along the edges, 9 to 12 minutes. If anything, pull these cookies a minute before you think they are done; they still taste fantastic slightly underbaked (with a gooey brownie-like texture inside). While they are still hot from the oven, sprinkle the tops of them with a little fleur de sel, if you like. Let the cookies cool on the baking sheets for 5 minutes. Serve warm or transfer the cookies to a wire rack and let them cool completely.
How to store: The cookies can be kept, tightly covered, at room temperature for up to 2 days.