I've not tried this...yet
ETA...I just made it. I had to cook it longer than 50 seconds, but I have a low wattage microwave. I did the 50 and then 15 and then another 15.
You know how a cookie can be crispy on the edges, but softer in the middle? Well, this was like that soft middle. It was good, but it was really chocolaty so I would cut it back to one tbsp of chocolate chips.
Yield: Makes 1 mug-full (1 large serving or 2 small servings to share)
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
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).
The cookie may not look completely cooked on top, but it is fully cooked below.
Either light or dark brown sugar work fine.