Mash the banana until mostly smooth, then mix in the egg, vanilla, and almond milk.
Stir in the almond flour, oat flour, sugar-free sweetener, baking powder, and cinnamon until fully combined.
Transfer the batter to a greased mug or ramekin filled no more than two-thirds full.
Microwave for about 90 seconds to 2 minutes, depending on your microwave, until the center is just set.
Cool and serve