Preheat oven to 350° and prepare your skillet by coating with butter or nonstick spray, then set aside.

Mix the cookie dough first. Beat together the butter, sugars, and vanilla until smooth, then beat in the egg. Add the dry ingredients and mix until combined, then stir in the chocolate chips. Drop scoops of cookie dough into the prepared skillet and press them so that the tops of the scoops are pretty even with each other. Set aside.

Prepare the brownie batter by beating the melted butter, sugar, vanilla, and egg, then stir in the dry ingredients. Pour the batter in around the cookie dough in the skillet to fill in the gaps between the dough scoops. You can shake the skillet a little to help the batter settle in to the cracks more evenly.

Place the brookie in the oven and bake for 23-25 minutes, until it is just set. It will look shiny and not quite done in the very center, but it will continue to cook a little after you've removed it. Let the brookie cool for a little while (an hour or so is best) before serving.