5 ounce(s) bittersweet chocolate (60-75% cacao), divided
2 tablespoons Dutch-process unsweetened cocoa powder, sifted
3 large egg whites
1/2 teaspoon cream of tartar
1/2 cup sugar, divided
1/2 teaspoons vanilla extract
Preheat to 350°F. Place racks in upper and lower thirds of oven. Line two cookie sheets with parchment paper and lightly grease with cooking spray.
Chop three ounces of chocolate and place it in a small microwave-safe bowl and microwave on medium for 1 minute. Stir, then continue microwaving on medium, stirring every 20 seconds, until mostly melted. Repeat until chocolate melts completely. Alternatively, chocolate can be melted over double boiler.
Chop the remaining two ounces chocolate into pieces the size of mini chocolate chips. Combine in a small bowl with cocoa.
Combine egg whites and cream of tartar in a clean medium bowl and 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 and continue beating for 1 minute.
Gradually add the remaining sugar, tablespoon at a time, and vanilla, continuing to beat on medium speed until the mixture is smooth, opaque, glossy and thickened, about 2 minutes. Scrape down the sides of the bowl, raise the speed to high, and beat for another 30 seconds. Lightly fold in the chocolate-cocoa mixture and the melted chocolate just until evenly incorporated and no streaks remain. Be careful not to over mix and deflate the meringue. 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, around 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. Use a wide-bladed spatula to lift the cookies from the parchment paper. Store Dark Chocolate Meringue Cookies in an air-tight container.