These round disks have a delicate crisp crunch, yet remain intensely soft and moist inside. Sifting the dry ingredients is important because you want the flour to be finely aerated. The batter needs to be refrigerated a few hours before baking, so give yourself leeway by making it ahead.
MAKES ABOUT 6 DOZEN COOKIES
2 cups all-purpose flour
2 cups confectioners’ sugar
1⁄2 cup cocoa powder, preferably Dutch-processed, sifted
2 teaspoons baking powder
1⁄2 teaspoon salt
4 ounces unsweetened chocolate, chopped
4 tablespoons (1⁄2 stick) unsalted butter, cut into small pieces
1 1⁄2 cups packed light brown sugar
1⁄3 cup light corn syrup
2 teaspoons vanilla extract
4 large egg whites, at room temperature
1. Spread a sheet of wax paper on your work surface. Sift the flour, 1 1⁄2 cups of the confectioners’ sugar, the cocoa powder, baking powder, and salt together, using a flour sifter or a fine-mesh sieve, allowing the sifted mixture to fall onto the wax paper. Repeat, making sure the cocoa powder is evenly distributed through the mixture. Set aside.
2. Place the chocolate and butter in the top of a double boiler or in a medium bowl that fits securely over a medium saucepan. Bring about 1-inch of water to a boil in the bottom of the double boiler or in the saucepan; place the top of the double boiler or the bowl over the simmering water, reduce the heat but maintain the boil, and stir until half the chocolate has melted. Remove the top of the double boiler or the bowl from over the water—be careful of any escaping steam—and continue stirring off the heat until all the chocolate and butter have melted and the mixture is smooth. Transfer to the bowl of a standing mixer, or to a large, clean bowl if you’re using a handheld mixer, and cool for 5 minutes.
3. Beat the brown sugar into the chocolate mixture, using an electric mixer at medium speed, until light and silky, about 1 minute. Beat in the corn syrup and vanilla. Scrape down the sides of the bowl, then beat in the egg whites all at once until smooth, a little less than 1 minute.
4. Remove the beaters and stir in the prepared flour mixture, using a wooden spoon or stiff spatula; stir just until any trace of flour has disappeared. Cover the bowl with a clean kitchen towel and set in the refrigerator until the mixture becomes firm, at least 1 hour but no more than 8 hours.
5. Arrange an oven rack in the center of the oven; preheat the oven to 350°F. Line a baking sheet with parchment paper or a silicone baking mat; set aside. Place the remaining 1⁄2 cup confectioners’ sugar in a small bowl; set aside as well.
6. Roll a small piece of the firm dough into a small ball, about the size of a large olive. Roll this ball in the confectioners’ sugar, then place it on the prepared baking sheet. Continue making sugared balls, spacing them about 11⁄2 inches apart, until the baking sheet is full. (It may be easier to make all the balls first, then roll them in the confectioners’ sugar—you are less likely to transfer confectioners’ sugar back into the chocolate dough.) Return any unused dough to the refrigerator.
7. Bake for about 16 minutes, or until the cookies have spread and cracked at the edges and are somewhat firm to the touch. Cool for 5 minutes on the baking sheet, then carefully transfer them to a wire rack using a metal spatula (they are still a little fragile because they’re so fudgy). Let the baking sheet cool for 5 minutes before baking additional batches; if using parchment paper, replace it with a new sheet. Let the baked cookies cool completely on the wire rack.
5 days at room temperature
Not recommended for freezing