Preheat the oven to 350 degrees. Use the convection bake setting if you have it. Grease a 11-13" skillet with baking spray and set it aside.
In a medium-large sized mixing bowl, use a hand or stand mixer to cream together the butter and brown sugar.
Add the milk, vanilla extract and egg. Mix until combined. Scrape down the bowl.
Add the salt and baking soda, then mix again until combined.
Add the flour, then mix until combined. Scrape down the bowl once complete.
Fold in ¾ cup of the chocolate chips, reserving ¼ cup for topping the skillet brookie.
Set the dough aside while you make the brownie batter.