When ready to bake, preheat oven to 350°F and grease a baking sheet with nonstick spray.
In a large mixing bowl, beat the butter, cream cheese, sugars, egg, and vanilla on medium-high speed with an electric mixer until light and fluffy, about 5 minutes.
Scrape down the sides of the bowl, add cocoa powder, and beat again on medium-high speed until incorporated.
Add flour, cornstarch, baking soda, and salt and beat until just combined.
Stir in chocolate chips and scoop dough out in 1/4 cup mounds. Roll into balls, flatten slightly, and place on baking sheet 2 inches apart.
Cover with plastic wrap and chill at least 2 hours.
Bake until edges have set but tops are still slightly glossy, about 10 minutes. Cookies will firm up a bit as they cool; be careful not to over bake.
Allow to cool on baking sheet for 10 minutes before serving. Enjoy!