Whisk together the dry ingredients- flour, baking soda, cornstarch, and salt.

In the bowl of an electric mixer, beat together cold butter cubes and sugars until light and fluffy. It will be crumbly at first, but should come together in 4-5 minutes!

Add in the egg and yolk and beat until smooth. Beat in the vanilla extract.

Stir in the dry ingredients until just combined.

Stir in 1/2 cup each of white chocolate chips, milk chocolate chips, and semi sweet chips. (see note below)

Refrigerate the dough for at least 2 hours or overnight. If refrigerating overnight, you'll need to take the bowl out of the fridge about a half hour before you plan to bake these so they're scoopable.

Preheat the oven to 400 degrees (I highly recommend an oven thermometer!). Line two baking sheets with parchment paper. Scoop the dough out into 2 Tablespoon size rounds. Place on the baking sheet, about 2 inches apart.

Bake for 6-8 minutes or until edges are browned, but centers are still soft and puffy. They should look slightly underdone in the center.

Remove from the oven and dot with remaining chocolate chips. Allow the cookies to set up for about 5-10 minutes or until they're set enough to transfer. Transfer to a cooling rack to cool completely. Cookies will stay soft for several days when kept in an airtight container.