Preheat & prep: Preheat oven to 375°F. Line two baking sheets with parchment paper or silicone mats.

Cream the butter & sugars: In a stand mixer fitted with the paddle attachment, beat the cold cubed butter, brown sugar, and granulated sugar on medium speed for 3-4 minutes, until slightly lightened in color and well combined. The texture will be like a paste, and not really light or fluffy.

Add wet ingredients: Mix in the eggs, scraping down the sides as needed. Add vanilla extract and mix until incorporated.

Combine dry ingredients: In a separate bowl, whisk together the flour, dark cocoa powder, cornstarch, baking soda, baking powder, and salt. (Careful with the dark cocoa, it can be messy! Haha)

Form the dough: Add the dry ingredients to the wet mixture and mix on low speed until just combined. Do not overmix. The dough will be thick and slightly tacky.

Fold in chocolate chips: Using a spatula, gently fold in both the white chocolate chips and semi-sweet/milk chocolate chips until evenly distributed.

Place on baking sheet: Grab a big mound of cookie dough, ~3-4 oz. and place on the cookie sheet, keeping the cookie dough tall. I like to almost pinch the dough with my fingertips to give the cookies a rustic bakery-style look.

Bake: Bake for 10-12 minutes, or until the tops are set but the centers are still slightly soft. (Tip: right after baking, if any cookies look wonky, just take the back of a spoon and gently press the cookie dough back into a circular shape. But they shouldn’t overly spread.)Start Timer

Cool & serve: Let cookies cool on the baking sheet for 10-15 minutes before transferring to a wire rack.