In a large bowl, sift together the flour, cocoa, cornstarch, baking soda and salt. Whisk together. Set aside.
In a separate large bowl, beat together the butter, brown sugar and granulated sugar until there are no lumps of butter or brown sugar. Turn off the mixer and scrape down the sides and bottom of the bowl.
Mix the eggs and vanilla extract into the butter mixture.
With the mixer on a low speed, beat the dry ingredients into the butter mixture. I typically do this about ½ at a time. Turn off the mixer and scrape down the sides and bottom of the bowl a few times to ensure there aren't lumps of butter stuck to the bottom.
Cover the bowl and refrigerate for at least 30 minutes or up to 48 hours.
When ready to bake, preheat the oven to 350℉ (180℃). Line the cookie sheets with parchment.
Scoop the dough into balls with about 1½ to 2 tablespoons of dough each. I weighed each of mine and made each 60 grams.
Bake in the middle of the preheated oven for about 11-13 minutes, or until the tops look set.
Remove from the oven. Right away, grab a water glass or some form of glass with a flat bottom. Use the flat bottom to gently press the top of the cookie down, so that the top is flatter. Continue to cool the cookies on the tray.