Preheat your oven to 400°F. In a large mixing bowl, cream together the cubed butter, sugar, and brown sugar for 3-4 minutes until light and fluffy. Scrape the sides of the bowl halfway through to ensure even mixing.
Add the eggs (and vanilla, if using) and mix for 1 more minute until smooth and creamy.
Fold in the dark cocoa powder, flour, cornstarch, baking soda, and salt. Mix until just combined. Fold in the white chocolate chips and milk chocolate chips.
Roll the dough into 6-ounce balls. Weighing the dough with a food scale ensures even baking. Place the dough balls on a light-colored baking sheet to prevent the bottoms from browning too much.
Bake for 9-11 minutes. After baking, let the cookies set for at least 10 minutes.