Preheat the oven to 375 degrees F and line baking sheets with parchment.

Place the butter, brown sugar and granulated sugar in a large mixing bowl and beat on medium-high speed until very pale and fluffy (3 to 5 minutes).  

Add the egg, egg yolk, and vanilla, beating until completely incorporated.  

Scrape the bottom and sides of the bowl with a silicone spatula, then add the flour, cocoa powders, cornstarch, baking soda, and salt, stirring on medium-low speed just until all the ingredients are combined (do not over-mix).  

Stir in the chocolate chips.

 Use a 1 1/2-tablespoon cookie scoop to drop balls of cookie dough onto the prepared sheets, allowing about two inches in between (for spreading).  

Bake (one sheet at a time) on the center rack of the oven for 7 to 10 minutes, or until set around the edges.  

Cool completely on the baking sheet, then transfer to an air-tight container for storage.