Position a rack in the center of your oven and preheat the oven to 350°F. Line 2 large baking sheets with parchment paper, and set aside.
In a large bowl, combine your powdered sugar, brown sugar, milk, and vanilla extract. Whisk well to combine the ingredients and dissolve the brown sugar in the milk. Set aside.
In a separate large bowl, combine flour, cornstarch, baking soda, and salt. Whisk to incorporate. Add unsalted butter to the dry mixture and cut the butter into the mix until you're left with small pieces of butter (no larger than a pea).
Pour your sugar / milk mix into your dry ingredient / butter mix. Stir until it comes together into a thick cookie dough. Add chocolate chips or chunks. Mix to incorporate.* If the dough is too sticky to work with, pop it into the fridge for 30 to 60 minutes, or until firm enough to handle.
Using an ice cream scoop, 2 spoons, or your hands, portion out the dough into approximately 12 balls. Drop the balls onto the parchment lined baking sheets leaving 1 to 2 inches of space between each cookie. Press each of the balls down into a disk shape. ** see notes
Bake the cookies in the center of a 350°F oven until golden brown on the edges and bottom (about 10 to 12 minutes). Cool the cookies for about 5 minutes on your baking sheets before transferring them to a wire cooling rack to cool completely.