- 5 ounces bittersweet chocolate (60-75% cacao), divided
- 2 tablespoons unsweetened cocoa powder (preferably Dutch-process), sifted after measuring if lumpy
- 3 tablespoons cocoa nibs (see Tip) (optional)
- 1/3 cup egg whites, at room temperature
- 1/2 teaspoon cream of tartar
- 1/2 cup sugar, divided
- 1/2 teaspoon vanilla extract
- Position racks in upper and lower thirds of oven; preheat to 350 degrees F. Line 2 baking sheets with parchment paper and coat the paper with cooking spray.
- Coarsely chop 3 ounces of chocolate and place it in a small microwave-safe bowl. Microwave on Medium for 1 minute. Stir, then continue microwaving on Medium, stirring every 20 seconds, until mostly melted. Stir until the remaining chocolate melts completely.
- Chop the remaining 2 ounces chocolate into pieces the size of mini chocolate chips. Combine in a small bowl with cocoa and cocoa nibs (if using).
- Combine egg whites and cream of tartar in a clean medium mixing bowl. Beat with an electric mixer on low for 30 seconds, then at medium speed until soft peaks start to form. Immediately add about 2 tablespoons sugar; beat for 1 minute. Slowly, about a tablespoon at a time, add the remaining sugar, then vanilla, continuing to beat on medium speed until the mixture is smooth, opaque, glossy and thickened, about 2 minutes longer. Scrape down the sides of the bowl, raise the speed to high, and beat for 30 seconds more. Lightly fold in the chocolate-cocoa mixture and the melted chocolate just until evenly incorporated and no streaks remain; do not over mix. Immediately drop the batter by rounded teaspoonfuls about 1 inch apart onto the prepared baking sheets.
- Bake the cookies, switching the pans back to front and top to bottom halfway through, until just firm when gently pressed on top but still soft inside, 8 to 12 minutes. Transfer the pans to wire racks and let stand for 1 to 2 minutes. Then slide the paper from the pans to a flat surface and let the cookies cool completely, about 15 minutes. Gently lift the cookies from the parchment paper using a wide-bladed spatula.