Whisk brownie mix, eggs, milk, vanilla, melted butter, and salt until smooth; fold in chips if using.

Heat ovenproof skillet over medium, add 1 tbsp butter to coat. Pour batter into skillet, smooth top.

Cook 3–4 minutes until edges set, then transfer to 350°F oven for 10–12 minutes until a toothpick near center comes out with moist crumbs.

Meanwhile beat softened butter with 2 tbsp maple syrup to make maple butter.

Let skillet rest 5 minutes, spread maple butter, slice and serve with extra syrup.