Preheat oven to 350 degrees F. Spray a baking sheet with nonstick spray, or instead use parchment paper or silpats.
Melt 1 cup chocolate chips in the microwave, stirring every 30 seconds until just melted. Cool slightly.
Using an electric or stand mixer, beat whites in large bowl to soft peaks (use the whisk if using a stand mixer).
Gradually beat in 1/2 cup powdered sugar. Continue beating on high until mixture resembles soft marshmallow creme.
Whisk cocoa, 1 cup powdered sugar, cornstarch, and salt in a medium bowl. On low speed, beat dry ingredients into the meringue. (I switched to the paddle attachment on my mixer here).
Stir in lukewarm chocolate and 1/2 cup chocolate chips. Dough will become very stiff.
Place 1/2 cup powdered sugar in a 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 for 5 minutes, then remove to cooling rack.
Notes
Source: Divine Baking, a blog that's not up anymore.