In a 12-ounce microwave-safe mug, add the flour, sugar, cocoa powder, baking powder and salt. Stir together with a small whisk or fork until combined.
In a small bowl, combine the mashed banana, milk, oil and vanilla. Stir until smooth, then pour into the mug with the dry ingredients and mix until the batter is just combined. Alternatively, add the wet ingredients directly to the mug and mix until just combined. Wipe down the outside of the mug with a damp paper towel to remove any spills before microwaving.
Place the mug in the microwave and heat on high for 60 to 90 seconds, checking at 60 seconds. The cake is done when the top looks mostly set with a small glistening spot in the center. You can also test if the mug cake is done by inserting a toothpick into the center of the cake. It should come out with just a few moist crumbs attached rather than wet batter. Let the mug sit in the microwave for 1 minute before carefully removing.
Allow the mug cake to cool for 1 to 2 minutes before topping with chocolate chips and banana slices. The center may be very hot, so take care with your first few bites.