Preheat oven to 350°F (175°C). Grease a 10‑inch ovenproof skillet or round baking pan.
Make brownie batter per box directions and pour into prepared skillet, smoothing the top.
In a bowl, cream butter with sugars, beat in egg and vanilla, then mix in flour, baking soda, and salt until just combined.
Fold in chocolate chips, drop cookie dough by spoonfuls over the brownie batter and gently spread to cover as evenly as possible.
Bake 25–35 minutes until cookie top is golden and a toothpick in the center of the brownie layer comes out with a few moist crumbs.
Cool 15 minutes before slicing; serve warm with ice cream if you’re feeling fancy.