Place butter and sugar in the bowl of a large stand mixer, fitted with a paddle attachment OR in a large mixing bowl with a wooden spoon. Mix on medium speed or by hand until creamy, about 2 minutes. Add egg and vanilla, beating well until smooth.
In a medium separate bowl, combine flour, cocoa, cornstarch, baking soda, and salt. Add to butter mixture, beating on medium speed until dough is just combined. Fold in chocolate chips.
Scrape dough into a ball and wrap in plastic wrap. Refrigerate dough 2 hours or up to 6 days.
When ready to bake, preheat oven to 350F. Line 2 baking sheets with parchment paper and set aside.
Remove dough from refrigerator and scoop dough out into 1-inch balls. Place balls 2 inches apart on prepared baking sheets and flatten slightly.
Bake cookies at 350F 10-12 minutes until edges of cookies are set (middles will be slightly soft). Allow cookies to cool on baking sheet 20 minutes before transferring to a wire cooling rack to cool completely. Enjoy!