Peel banana half and mash in a small bowl. (You can reserve the rest of the banana to slice as garnish, if it is not too mushy.)
Scrape mashed banana into a microwave-safe mug, then stir in vegetable oil, milk, brown sugar, flour, baking powder, cinnamon, and salt.
Stir in chocolate chips.
Scrape any batter on the sides of the mug down into the rest of the batter.
Microwave for 80 seconds on high.
Top with banana slices and more chocolate chips, if desired.
Notes
Expert Tips: You may need to test this recipe in your own microwave a couple of times before you get the timing right, since microwaves can be very different. When the mug cake is fully baked, the surface of the cake will look mostly dry with just a small area or two that looks a little wet. The cake will continue to cook as it is cooling. If you like your cakes more gooey in the middle, you may want to take a few seconds off the cook time. We recommend a ceramic mug with straight sides and a narrow opening, which helps the cake build volume. But with a tall, skinny mug, it can be hard to make sure there is not a chunk of flour on the bottom. Make sure you mix the batter well so that all ingredients are incorporated.