Preheat the oven to 350 degrees. Line cookie sheets with parchment paper or silpat baking mats.
Cream butter, shortening, and sugars together in an electric mixer fitted with a paddle attachment. Beat in egg, vanilla, and milk. Beat in salt and baking soda. Stir in flour and mix until combined. Stir in chocolate chips.
Scoop 1 tablespoon balls of dough (I use a small cookie scoop for this) and place onto cookie sheets. Press the dough lightly with the palm of your hand to flatten just a bit (about half the thickness of the ball). Bake for about 13 minutes, until the cookies are slightly browned. Remove from oven. Cool 5 minutes on cookie sheet before removing to rack to cool completely. They will get crunchier as they cool.