These cookies are sublime! They are so popular I tried to make a double batch once – but you get a lot of yolks left over!
Nonstick vegetable oil spray
1 1/2 cups bittersweet chocolate chips (about 9 ounces), divided
3 large egg whites, room temperature
2 1/2 cups powdered sugar, divided
1/2 cup unsweetened cocoa powder
1 tablespoon cornstarch
1/4 teaspoon salt
Preheat oven to 400°F. Spray 2 large baking sheets with nonstick spray. Melt 1 cup chocolate chips in glass bowl in microwave, stirring twice, about 2 minutes. Cool slightly.
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 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.