In a large bowl, whisk together the flour, cocoa powder, cornstarch, baking powder, baking soda and salt. Set this aside. In the large bowl of a stand mixer fitted with a paddle attachment, beat the butter and both sugars together until smooth, then add the egg, egg yolk and vanilla and beat until combined, scraping down the sides of the bowl as needed.

Add the dry ingredients to the wet and mix on low speed until combined, scraping down the sides of the bowl as needed. Lastly, beat in the chocolate chips just until they are incorporated into the dough. Cover the dough tightly with plastic wrap and chill it in the fridge for at least 30 minutes.

Preheat the oven to 350°F. Line a few large baking sheets with parchment paper or silicone baking mats. Roll the dough into balls, about 1 tbsp of dough per ball and place on the trays about 1 inch or so apart.

Bake for 8-10 minutes. The centers will still look very underbaked with the edges set and that's how you want them to be. If you'd like, immediately press additional chocolate chips gently into the tops of the cookies. Let them cool for about 10 minutes on the baking tray, them remove them to a wire rack to cool completely.