9 ounces dark or bittersweet chocolate chips, divided [BULK]
3 large egg whites, at room temperature
2 1/2 cups powdered sugar, divided [BULK]
1/2 cup Dutch-processed unsweetened cocoa powder [BULK]
1 tablespoon cornstarch [BULK]
1/4 teaspoon salt
1. Preheat oven to 400°F. Line 2 large baking sheet with parchment paper. Melt 1 cup chocolate chips over a double boiler or in a glass bowl in the microwave, stirring twice, about 2 minutes. Cool slightly.
2. Using electric mixer, beat egg whites in large bowl to soft peaks. Gradually beat in 1 cup powdered sugar. Continue beating until mixture resembles soft marshmallow crème.
3. Whisk 1 cup powdered sugar, cocoa, cornstarch, and salt in medium bowl to blend. On low speed, beat dry ingredients into meringue. Stir in lukewarm chocolate and remaining 1/2 cup chocolate chips (dough will become very stiff).
4. Place remaining 1/2 cup powdered sugar in a shallow bowl. Roll 1 rounded tablespoon dough into ball; roll in sugar, coating thickly. Place on prepared sheet. Repeat with remaining dough, spacing 2 inches apart. Bake until puffed and tops crack, about 10 minutes. Cool on sheets on rack 10 minutes. Transfer to rack; cool slightly. These are best eaten while they are still warm.