Brown the butter in a saucepan until golden and nutty; cool 10-20 minutes.
1/2 cup ½ cup unsalted butter
Whisk butter with brown sugar and granulated sugar until glossy.
1/2 cup light brown sugar, 1/4 cup granulated sugar
Whisk in egg and vanilla until smooth.
1 large egg, 1 tsp vanilla extract
Stir in flour, cornstarch, baking soda, and salt just until combined.
1 cup all-purpose flour and 2 tablespoons, 1½ tsp cornstarch, 1/8 tsp baking soda, ½ tsp salt
Fold in chocolate.
3/4 cup chocolate chips or chopped chocolate
Press evenly into a lightly greased 8-inch skillet, smoothing the top flat.
Bake at 325°F for 24–28 minutes, until edges are set and the center is slightly soft.
Rest 15 minutes before serving; sprinkle flaky salt on top.
Flaky salt for finishing