Place the unsalted butter in an 8-ounce microwave-safe mug. Microwave on high for about 30 seconds until the butter is fully melted.
Add the egg yolk and mix until well combined. Stir in the all-purpose flour and the chocolate chips, then sprinkle additional chips on top.
Microwave on high until the cookie puffs slightly and appears set, about 45 to 60 seconds. Let cool for about 1 minute before enjoying.