Little update: I realized after I posted this (and tried it with this variation) that if you are making more than one at a time, try using a whole egg, whisking it, then splitting the egg mixture between the two recipes/mugs. That way you aren't wasting two egg whites. It works just fine that way - give or take, it's about a tablespoon of egg per mug.
Also, to avoid the batter being crumbly (thanks to the commenters who stumbled upon this), mix all the wet ingredients first: butter, egg yolk, sugars and salt. Then gradually add the four and stir in the chocolate chips.
1 tablespoon melted butter or coconut oil
1 tablespoon granulated sugar
1 tablespoon brown sugar
pinch of salt
1 egg yolk
1/4 cup all-purpose flour
2 tablespoons chocolate chips (regular size or mini)
Very lightly coat the inside of a 6- or 8-ounce mug or ramekin with a smidgeon of cooking spray, butter or coconut oil. Mix all the ingredients together in the mug (to avoid the batter being crumbly, mix the butter, egg yolk, sugars and salt first before gradually stirring in the flour and adding the chocolate chips).
Microwave for one minute at 80% power (I have a 1,000 watt microwave; if your microwave has more or less watts, you may need to adjust the time a bit). The top will look light and yellow and may even seem undercooked but if you gently press the top and it lightly springs back, it's perfect (don't overcook or it will be dry and tough in spots).
Serve warm with a scoop of vanilla ice cream, if desired.