Begin by melting the unsalted butter in a saucepan over medium heat until it turns golden brown and smells nutty, about 5–7 minutes. Chill for 1 hour after cooling slightly.

In a large mixing bowl, combine the chilled brown butter, granulated sugar, and light brown sugar. Beat until creamy, about 2–3 minutes. Mix in the egg, then incorporate flour and chocolate chips.

Melt the bittersweet chocolate in a separate saucepan over low heat. Stir in the remaining butter, sugar, and egg until glossy. Fold in a pinch of flour.

Grease an 8 x 8 inch baking pan. Spread half of the brownie batter at the bottom, dollop cookie dough over it, then add the remaining brownie batter. Swirl gently to marble.

Preheat oven to 350°F (175°C). Bake for about 25 minutes until edges are golden and center is soft. Use foil if edges brown quickly.

Cool completely in the pan, then refrigerate for 30 minutes before slicing into squares.