Mash the banana in a microwave-safe mug until smooth and creamy.
Add the egg, milk, and sweetener. Stir well to combine.
Add the flour, baking powder, cinnamon, and salt. Mix until fully combined.
Fold in chopped nuts for texture and added flavor.
Microwave on high for 2 to 3 minutes until the cake is cooked through.
Let it cool for 1 minute before serving warm with your favorite toppings.
