In a large, microwave-safe or oven-safe mug, combine the mashed banana with the egg, oil, maple syrup, and almond butter and mash with a fork until smooth. Still using the fork, whisk in the vanilla, flour, arrowroot, baking powder, and salt until smooth. Stir in the chocolate chips. If the batter reaches higher than halfway up the sides of the mug, divide the batter between two mugs. Gently place the banana slices on top of the batter.