Both icing sugar, salt and cocoa are prone to clumping so it is important to sift them through a sieve before using them in this recipe.
Sift together the icing sugar, cocoa and corn starch.
Mix in the egg whites until smooth.
Blend in the vanilla extract and vinegar until smooth, then fold in the chocolate chips.
Preheat oven to 350 degrees.
Line a couple of cookie sheets with parchment paper or silicone liners.
Spoon the cookie batter by rounded teaspoons onto the prepared cookies sheets, about 2 to 3 inches apart. These spread quite a bit so give them plenty of room. (You can sprinkle on a few extra chocolate chips at this point if you like. It makes them look even more appealing when baked.
Bake for approximately 15 minutes until the cookies have a crackled appearance at the edges and might still look very slightly shiny at the center. It is best to do a test bake of a couple of cookies tot get the baking time right, as is suggested in the NOTE in this recipe.
These cookies will be very hot and soft when they come out of the oven, so be sure to let them cool completely on the pan for about 15-20 minutes before transferring them to a wire rack to cool completely.
