Deep Dark Chocolate Cookies, from Divine Baking, who adapted it from Epicurious
1 1/2 c bittersweet chocolate chips (about 9 oz), divided into 1 c & 1/2 c
3 large egg whites, room temperature
2 c + powdered sugar, divided into 1 c & 1 c
1/2 c unsweetened cocoa powder
1 T cornstarch
1/4 t salt
Preheat oven to 350°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 a little extra 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.