Add the egg and vanilla extract, then beat again. Remember to scrape down the sides and bottom of the bowl.
Incorporate the flour, dark cocoa powder, cornstarch, baking soda, and salt; beat until well-combined. Scrape down the sides and bottom of the bowl once more.
Mix in the chocolate chips/chunks, ensuring they are evenly distributed.
Take a portion of the dough and measure out 100 g. Form it into balls and place them on the prepared baking sheet or a smaller one, then freeze for approximately 15 minutes.
Bake in the preheated 400°F oven for 8-12 minutes.
Allow the cookies to remain on the baking sheet for 15 minutes before indulging in them.