Preheat oven to 350f. combine flour, cornstarch, salt and baking soda in small bowl and set aside. add the sugars and butter to the bowl of a stand mixer and cream until foully. add egg and vanilla and mix again until combined. add dry ingredients and mix until flour is fully incorporated. divide the dough into two equal balls. remove one half of the dough from the mixer and set aside. add in 1 cup chocolate chips into the mixer and mix until chips are evenly distributed 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 until cocoa is completely blended into dough. add in 1 cup chocolate chips and mix until chips are evenly distributed in the dough. wrap chocolate dough into plastic wrap. place both dough flavors into refrigerator and chill for at least 1 hour. line baking sheets with parchment. using a spoon, 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 10-12 minutes, until middle is set and edges are starting to turn brown.