Preheat oven to 350°F.
Place a 9 or 10-inch cast iron skillet over medium heat and add the butter. Melt, swirling occasionally, until it turns golden brown and smells nutty. Remove from heat.
Once the butter has cooled, in a medium bowl, combine the butter, brown sugar, and granulated sugar. Beat with a hand mixer or whisk until smooth and glossy.
Stir in the egg, milk, and vanilla until fully combined.
Sprinkle the flour, salt, baking soda, and baking powder evenly over the mixture. Gently mix everything together in the skillet until a soft dough forms (don’t overmix).
Fold in the chocolate chips and/or chopped chocolate until evenly distributed.
Press the dough into the cooled cast iron skillet used to brown the butter in an even layer. Bake 20–25 minutes, or until the edges are golden and the center is mostly set (it will continue to cook slightly as it cools).
Sprinkle flaky sea salt over the top while warm. Add additional chocolate on top while still warm, if desired.
Cool for about 5-10 minutes before serving with desired toppings.