Mix 2 tablespoons butter, 2 tablespoons sugar, 1 tablespoon brown sugar, and 1 egg yolk in a microwave-safe mug or bowl
Add 1/4 teaspoon vanilla extract and mix well
Stir in 4 tablespoons all-purpose flour and a pinch of salt
Add 1 to 2 tablespoons chocolate chips
Flatten the dough slightly in the mug or shape small cookie mounds on a microwave-safe plate lined with parchment paper
Microwave on high for 40 to 60 seconds
Check for doneness; microwave in 10-second intervals if needed
Let the cookies cool for 1 to 2 minutes before eating