Microwave butter in a mug until melted, 30 seconds to 1 minute. With a spoon, mix in sugars, vanilla extract and salt until well-combined. Mix in egg yolk until no traces of the yolk are seen. Mix in flour. Fold in chocolate chips.
Microwave on high for 40 seconds, or a maximum it 50 seconds.
Serving suggestion: serve warm with a dollop of ice cream