Spray a 12-ounce microwave-safe mug lightly with cooking spray or rub with a small amount of butter to prevent sticking.
Add the mashed banana and beaten egg to the mug, stirring with a fork until completely combined and smooth with no streaks of egg visible.
Add the flour, cinnamon, baking powder, vanilla extract, and salt directly to the mug. Stir thoroughly for 20-30 seconds until the batter is uniform and no dry pockets of flour remain.
Tap the mug gently on the counter 2-3 times to release any air bubbles and level the batter surface.
Microwave on high power for 50-70 seconds, watching through the window as it rises. Stop when the top looks just set but still slightly glossy in the center.
Let the mug sit for 30 seconds to finish cooking with residual heat. The cake will firm up and pull slightly away from the sides.
Run a butter knife around the edge if needed, then enjoy straight from the mug or turn out onto a plate. Serve immediately while warm and fluffy.
