Add the butter to a large mug and microwave until melted.
Remove from the microwave and whisk in the brown sugar (I use a fork for this).
Ensure that the butter isn't too hot (otherwise, you'll scramble the egg), then whisk in the egg yolk and vanilla extract.
Stir in the flour (the mixture should be thick like cookie dough), then stir in the chocolate chips.
Microwave on medium power (50-70%) for 60-90 seconds or until the top looks just set. Do not microwave on high power, otherwise, the cookie can become hard and the chocolate can burn. I check after 60 seconds to see, then add an extra 15 seconds as needed.
Enjoy immediately, optionally with a scoop of ice cream.