Whisk dry ingredients (flour, cornstarch, and cocoa powder) together and set aside.
Cream butter and sugars with paddle attachment, on medium-low speed, until fluffy and pale in color (about 5 minutes).
Add egg and vanilla extract and mix on low speed until thoroughly combined. Scrape down the paddle with a spatula, and then continue to mix on low speed for another 30 seconds to ensure everything is fully incorporated.
Add dry ingredients, all at once, and mix on low speed just until dough starts to come together and no dry bits remain. Do not overmix.
Turn dough out onto cling wrap, tightly wrap, and refrigerate overnight (or for at least 4 hours).
Knead, roll, cut out cookies, and return to fridge or freezer until firm. Leave a couple inches between cookies.
Bake at 350°F for 15 minutes, rotate pan, and bake for another 1-5 minutes until edges are starting to darken in color.*
Remove cookies from oven and allow to cool on cookie sheet before handling.