Place the butter in an 8-ounce microwave-safe mug, and microwave for 30 seconds until mostly melted.
Stir in the sugar, vanilla, and salt with a spoon until combined. Add the egg yolk and stir until smooth. Stir in the flour and most of the chocolate chips until combined. Sprinkle the top with the remaining chocolate chips.
Microwave on high for 45 seconds to 1 minute or until the dough has puffed and appears dry. Let cool for a few minutes before serving. Best is enjoyed warm with ice cream.
Video
Notes
If you do not have a microwave-safe mug, you can use a ramekin or small microwave-safe bowl.
Make sure you use a large enough mug. The cookie inside of the mug rises more than a traditional cookie as it cooks, so you don’t want it to overflow inside the microwave.
Fluff, spoon, and level your flour with a knife to keep from overpacking your measuring spoon with flour. Too much flour will lead the mug cookie to become dry.
Not all microwaves are the same. I recommend keeping an eye on your mug to make sure you don’t accidentally overcook the cookie. Do not microwave for more than one minute.
I like using real vanilla extract instead of artificial extract for the best flavor. Try my homemade vanilla extract if you want an easy DIY project.
A shorter, wider mug is ideal as it allows for more even cooking. I recommend a heavy ceramic mug with no metal.