Preheat oven to 350°F. Lightly butter a 10-inch oven-safe skillet.
Cream butter and both sugars in a bowl until fluffy, about 2 minutes.
Beat in the egg and vanilla until smooth and glossy.
Whisk flour, cornstarch, baking soda, baking powder, and salt in a separate bowl.
Stir dry ingredients into the wet mixture just until a soft dough forms.
Fold in the chocolate chips, reserving a small handful for the top.
Press dough evenly into the prepared skillet. Scatter reserved chips on top.
Bake until edges are set and center is still soft, about 13–15 minutes.
Cool 5 minutes. Sprinkle flaky sea salt if using. Serve warm with ice cream.