Heat oven to 375, spray 2 large baking sheets with cooking spray or line them with parchment.
Melt 1 cup of chocolate chips in microwave, stirring occasionally, until they are mostly melted, about 2 minute. Stir until fully melted, then leave them to cool.
Using an electric mixer with a wire whisk attachment, beat the whites on high speed until they form soft peaks. Gradually beat in 1 cup of the sugar, and continue beating until mixture gets thick and glossy.
Whisk together 1 cup of sugar, the cocoa, cornstarch and salt in a medium bowl and blend. On low speed, beat the dry ingredients into the meringue with vanilla.
Now, with a wooden spoon, stir in the lukewarm chocolate and remaining 1/2 c chocolate chips. The dough will be batter-like at first and then, as you stir, will get very stuff.
Place the remaining 1/2 cup of sugar in a bowl. Roll 1 rounded tablespoon into a ball, then roll it in the sugar to coat it thickly. Place on prepared sheet, and repeat with the remaining dough, spacing balls 2 inches apart.
Bake until the cookies are puffed and the tops crack,about 10 minutes. Cool on the sheets on a rack 10 minutes, then transfer the cookies to a rack to cool completely.