Place butter in your mug and melt in the microwave (about 20 seconds.)
Stir in small pinch of salt white and brown sugar and vanilla extract.
Separate egg yolk from egg white and just add the egg yolk to the mug (discard remaining egg white or refrigerate it in an airtight container to use for later.)
Stir well, then add all-purpose flour.
Stir again and then add chocolate chips.
Stir one final time.
Put the mug in the microwave and cook for approximately 40 seconds.
Cookie will continue to cook when you take it out. Let it cool slightly then dig in! Or better yet, add a little ice cream on top! No baking soda or baking powder is needed. Also, there is enough butter in this recipe to keep it from sticking to the inside of the mug. This can also be made in a ramekin or small microwave-safe bowl.