Preheat oven to 180°C. Lightly grease your skillet.
Beat the very soft butter together with both sugars until combined. Add the beaten half egg and vanilla extract and mix until smooth. Fold in the flour, baking powder, and salt until a soft dough forms. Stir in the chocolate chips. Place the bowl in the fridge to chill while you prepare the brownie batter, this firms up the dough so it is much easier to shape and press into an even layer on top.
Make the brownie batter. Melt butter and dark chocolate together. Whisk in both sugars and egg and beat vigorously until the mixture turns lighter in color and ribbony — this creates the crinkle top. Fold in cocoa powder, flour, and salt.
Pour brownie batter into the greased skillet and spread evenly. Take chilled cookie dough from the fridge, break into pieces, and press gently into an even layer on top.
Bake for 18–20 minutes until the cookie top looks golden. Cover loosely with foil or baking paper and continue baking for a further 12–15 minutes.
Rest for 5 minutes before scooping directly from the skillet.