Mix the butter and sugar: In a microwave-safe mug, combine softened butter and brown sugar. Stir until smooth.

Add vanilla and egg yolk: Stir in the vanilla extract and the egg yolk until fully incorporated.

Incorporate flour and salt: Add the flour and pinch of salt. Mix until a soft, thick cookie dough forms.

Fold in chocolate chips: Stir in the chocolate chips evenly. Save a few to sprinkle on top for presentation.

Microwave: Cook in the microwave for 40–60 seconds. Start with 40 seconds and add more if needed. The cookie should be soft and slightly gooey in the center.

Cool slightly: Let it rest for a minute or two—it’s hot!