1 1/2 cups bittersweet chocolate chips (about 9 ounces), divided
3 large egg whites, room temperature
2 cups powdered sugar, divided
1/2 cup unsweetened cocoa powder
1 tablespoon cornstarch
1/4 teaspoon salt
Using electric mixer, beat whites in large bowl to soft peaks. Gradually beat in 1 cup sugar. Continue beating until mixture resembles soft marshmallow creme. Whisk 1/2 cup sugar, cocoa, cornstarch, and salt in medium bowl to blend. On low speed, beat dry ingredients into meringue. Stir in lukewarm chocolate and 1/2 cup chocolate chips (dough will become very stiff).
Place 1/2 cup sugar in 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.