Pick a mug of your choosing and melt the butter in it by microwaving for about 20 seconds. Check to make sure it doesn't boil.
Mix in the two sugars, vanilla, and salt.
Mix in the egg yolk.
Add the flour. Mix until it's fully incorporated.
Fold in the chocolate chips of your choosing. You can add more or less depending on how much you want. Pat the dough down so it's flat. Microwave for 45-50 seconds. It should feel springy if you touch it.
Eat hot out of the microwave. Enjoy!
*If you want an eggless, vegan version just add a flax egg in place of the egg yolk. It's 1 teaspoon of ground flax mixed into 2 teaspoons of water. It works well as an egg replacer.