Preheat oven to 300 degrees F (150 degrees C). Line 2 large baking sheets with parchment paper.
In a small bowl, whisk together sugar and cornstarch; set aside.
In a large bowl beat egg whites with an electric mixer, until foamy. Beat in vinegar on medium speed until soft peaks form (about 2 minutes). On high speed, beat in sugar mixture, 1 tablespoon (15 mL) at a time, until stiff glossy peaks form (about 2 to 3 minutes). Fold in chocolate chips and cocoa powder, until just blended.
Drop by tablespoonfuls (15 mL), about 1-inch (2.5 cm) apart, onto prepared baking sheets. Bake for 30 minutes or until cookies are dry to touch, rotating pans halfway through. Remove cookies from sheets and place on cooling racks. Sprinkle with cocoa powder, if desired. Cool completely. Store in airtight containers.
