Place butter or margarine in a mug and melt in the microwave for 1 minute.
Add sugars, vanilla and salt to the mug. Stir to combine. Add egg yolk and stir.
Add flour, then stir again. Add chocolate chips and mix once more. The mixture should look like cookie dough.
Microwave on high 40-60 seconds. Serve warm.
Tip: Don’t be afraid to play around a little. You can add in some cocoa powder to make a double chocolate chip cookie, or other toppings, such as sprinkles, white chocolate chips, or nuts.