Preheat oven to 350°F (175°C). Grease 10‑inch ovenproof skillet or round baking pan.
Prepare brownie batter per box; pour into prepared skillet and smooth top.
Cream butter and sugars in bowl; beat in egg and vanilla. Mix in flour, baking soda, and salt just until combined.
Fold in chocolate chips; spoon cookie dough over brownie batter and gently spread as evenly as possible.
Bake 25–35 minutes until cookie top is golden and toothpick in brownie-layer center has a few moist crumbs.
Cool 15 minutes before slicing; serve warm with optional ice cream.