Preheat oven to 350°F.
Lightly butter a 6-inch cast iron skillet.
Using a hand mixer, cream the butter and sugars until smooth.
Mix in the egg yolk and vanilla.
Stir in flour, baking soda, baking powder, and salt until just combined. (May appear crumbly, but it should come together easily when you press with your hands.)
Fold in the chocolate chips.
Press the dough evenly into a skillet. Add extra chips, or chocolate chunks on top.
Bake for 15-20 minutes,or until the edges are golden and center soft.
Sprinkle it with flaky sea salt and let it rest for 10–15 minutes before enjoying!
Serve warm with ice cream.