Preparation : 2 minutes
Cooking : 1 minute
Yield : Makes 1 full mug (1 large serving or 2 small servings to share)
Microwave butter in a mug until melted, 30 seconds to 1 minute.
With a spoon, mix in sugars, vanilla extract, egg yolk and salt until well-combined. Mix in flour. Fold in chocolate chips.
Microwave on high for 40-50 seconds, or a maximum of 50 seconds.
Serve warm (maybe even with a scoop of ice cream!).