In a microwave-safe mug, combine the melted butter, granulated sugar, and light brown sugar. Stir until well mixed.
Add the vanilla extract, salt, and egg yolk to the mug. Stir until fully combined.
Add the all-purpose flour to the mug and mix until a smooth dough forms.
Fold in the semisweet chocolate chips evenly throughout the dough.
Microwave the mug on high for 40 to 60 seconds, watching closely to avoid overcooking. The cookie should look set but still soft and chewy.
Allow the cookie to cool for 1 to 2 minutes before eating directly from the mug.