Cornstarch is used to make the cookie more tender and give them that melt in the mouth taste.
You could substitute no more than 20% of the flour with cocoa. Adding too much cocoa could alter the taste and texture of the baked good. So with this in mind I went to work.
Add some chocolate chips or chopped nuts to make them even more decadent. Or add a few sprinkles to bring in the holiday spirit.