How to Make It
In a 12-ounce mug, melt the butter in a microwave oven. Stir in both sugars along with the egg yolk and vanilla until no streaks of yolk remain. Add the flour, chocolate chips and kosher salt and stir until incorporated.
Microwave the cookie at high power for 1 minute, until the top is set and the cookie is slightly pulling away from the side of the mug. Let the cookie cool for 5 minutes.
Run a knife around the edge of the cookie and invert it onto a plate. Top the cookie with a pinch of flaky sea salt.