Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Whisk until smooth and well combined. Add the egg, egg yolk, and vanilla extract, mixing until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and sea salt.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.
Gently fold in the chocolate chips.
Pour the cookie dough into your preheated skillet, spreading it out evenly.
Place the skillet in the oven and bake for about 25-30 minutes, or until the edges are golden and the center is set but still soft.
Once out of the oven, sprinkle with flaky sea salt.
Allow to cool in the skillet for a few minutes before slicing. Serve warm, preferably with ice cream.
