Gather ingredients and preheat oven to 350. Spray 2 large baking sheets with nonstick spray.
Melt 1 cup chocolate chips in glass bowl in the microwave, stirring twice, about 2 minutes. Cool slightly.
Using an electric mixer, beat egg whites in large bowl to soft peaks. Gradually beat in 1 cup sugar. Continue beating until mixture resembles soft marshmallow cream.
Whisk 1 cup sugar, cocoa, cornstarch, and salt in medium bowl to blend. On low speed, beat dry ingredients into meringue.
Mix in lukewarm chocolate mixture and 1/2 cup chocolate chips. The dough will become very stiff.
Place 1/2 cup powdered sugar in bowl. Roll 1 rounded tbsp dough into ball; roll in sugar, coating thickly.
Place on prepared sheet. Repeat with remaining dough, spacing 2 inches apart. Bake until puffed tops crack, about 10 minutes. Cool on sheets 10 minutes before transferring to a wire rack.