1 tablespoon unsalted butter
1 tablespoon granulated sugar
1 tablespoon brown sugar, firmly packed
1/2 teaspoon vanilla extract
1/8 teaspoon (pinch) salt
1 egg yolk
3 tablespoons all-purpose flour
1-2 tablespoons chocolate chips
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 of 50 seconds. Serve warm (maybe even with a dollop of ice cream).