Place butter in a large microwave safe coffee mug. Microwave until just melted (don't let get very hot) about 30- 60 seconds. (It melts faster in a wide mug and takes a bit longer in tall, narrow mug).
Remove mug from microwave and add flour, dark brown sugar, milk, baking powder and salt. Stir well with a fork, until smooth. Stir in 2 ½ tablespoons chocolate chips.
Microwave (on high) for 1 minute and 20 seconds for a wide mug (as shown in the picture) or 1 minute and 30 seconds for a tall, narrow mug. Cooking longer might make the cake drier!
Remove from microwave, sprinkle more chocolate chips and salt.
Eat chocolate chip cookie mug cake hot with a tall glass of cold milk! (This mug cake is intended to be enjoyed hot. As with butter based mug cake recipes, It tends to get a little dry as it sits)
Video
Notes
Dark brown sugar is the key in getting the deep molasses chocolate chip cookie taste. Do not use light/golden. Eat HOT- This mug cake is intended to be enjoyed hot. As with butter based mug cake recipes, It tends to get a little dry as it sits.