Add the butter, sugars, vanilla and egg yolk to a microwave safe dish like a ramekin or mug. (A flat 4-inch/8oz ramekin will bake more evenly than a mug, just FYI.) Mix well.
Add the flour, salt, and chocolate chips. Mix and flatten the top evenly. Top with extra chocolate chips, if desired.
Cook in the microwave for 40 to 60 seconds until just set. Time will vary depending on the dish used and power of the microwave. Enjoy warm!