Add the sugars and butter to the bowl of a stand mixer. Cream on high speed until fluffy. Add in egg and vanilla and mix again on medium speed until egg is fully mixed in. Add in flour, cornstarch, baking soda. On very low speed, mix until dough comes together and flour is fully incorporated.
Divide the dough into two equal halves. Remove one half of the dough from the mixer and set aside. Add in 3/4 cup chocolate chips into the mixer and mix on low speed for a few seconds until chips are evenly spread out into the dough. Remove dough from mixer and wrap in plastic wrap.
Take remaining dough half and place into mixer. Add in cocoa powder and mix on low to medium speed until cocoa is completely blended into dough and dough is uniform in color. Add in 3/4 cup chocolate chips and mix on low speed for a few seconds until chips are evenly spread throughout the dough. Wrap chocolate dough into plastic wrap. Place both dough flavors into refrigerator and chill for at least 1 hour.
After dough is chilled, line two large cookie sheets with silicone mats. Preheat oven to 350°F.
Using a 1.5 tbsp cookie scooper, scoop as many dough balls as you can out of the vanilla dough. When all the dough is used up, repeat with the chocolate dough.
Take 1 chocolate dough ball and 1 vanilla dough ball and briefly roll them together to form one smooth ball, with one half mainly chocolate and the other half mainly vanilla. Place dough ball onto cookie sheet. Repeat with remaining dough, spacing about 2 inches apart. Press gently down on each ball so that it becomes almost like a disc.
Bake for 11-13 minutes, until middle is set and edges are starting to turn brown. Let cookies cool and set before removing them from the baking sheets.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.