Brown a stick of butter until just past amber-colored. Cool to room temperature.
Whisk the brown butter into the sugars.
Mix in the vanilla and egg until smooth.
Fold in the flour, salt, baking soda, and cinnamon until mostly incorporated. Stop mixing when there are still some dry spots.
Add in the chocolate and finish mixing until everything is fully incorporated.
Spread the cookie dough into a cast-iron skillet. Top with extra chocolate.
Bake for 15 to 25 minutes depending on how gooey you want the center. 15 minutes gets you an underbaked cookie dough center. 25 minutes gets you a fully-set but still gooey center. You can go to 30 minutes for a fully cooked cookie.
Serves 8 good servings or 1 if you're really committed. Top with vanilla ice cream for a real treat!