Preheat oven to 350 degrees F. Line a 13x9" rectangular baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray; set aside briefly.
For the cookie: beat together the butter, sugars, and vanilla in a medium bowl with an electric mixer, beating until creamy. Add in the egg and beat until fluffy, about 1 minute. Add in the dry ingredients and beat on low speed until a dough comes together. Stir in the chocolate chips, then spread the cookie layer evenly into the bottom of the pan. Set aside.
For the brownie: Stir together the melted butter, sugar, and vanilla together in a medium bowl until blended. Add in the eggs and mix well. Stir in the dry ingredients until batter just comes together. Pour the batter evenly over the cookie layer.
Bake for approximately 20-30 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Do not over-bake these as it will lead to dry, crumbly bars. You want them to be slightly underdone as they'll continue to cook as they cool down.