In a microwave-safe mug, combine the melted butter, granulated sugar, and brown sugar. Stir until well mixed.
Add the vanilla extract, salt, and egg yolk to the mug. Stir until fully incorporated.
Add the all-purpose flour to the mug and mix until a smooth dough forms.
Fold the chocolate chips into the dough evenly.
Microwave the mug on high for 40 to 60 seconds, or until the cookie is set but still soft in the center. Do not overcook to maintain chewiness.
Allow the cookie to cool for 1 to 2 minutes before eating directly from the mug.