In a microwave-safe mug, add the melted butter, brown sugar, and granulated sugar. Stir well until combined.
Add the vanilla extract, salt, and egg yolk to the mug. Mix thoroughly until smooth.
Gradually add the all-purpose flour to the mug, stirring continuously until a dough forms and no flour streaks remain.
Fold in the chocolate chips evenly throughout the dough.
Microwave the mug on high for 40 to 60 seconds. The cookie should look set but still soft in the center for a chewy texture.
Allow the cookie to cool for 2 minutes before eating. Enjoy directly from the mug.