Microwave butter until just melted, then cool.
Add sugars, whisk with a fork until smooth.
Add yolk and vanilla, mix until creamy.
Stir in baking soda, cornstarch, flour.
Add chocolate chips.
Roll into 3 balls or 1 large one.
Bake at 350°F for 10–12 minutes.
Sprinkle with sea salt, rest 5 minutes before serving.
