Pre-heat oven 350F/180C. Line muffin pan with muffin cases.
Mix bananas and margarine with a wooden spoon until combined.
Add sugar, egg, and vanilla mix well.
Sprinkle in baking soda and salt and mix.
Slowly add flour, blending after each addition.
Fill muffin cases 3/4 full, sprinkle cinnamon sugar on top and bake 18-25 minutes. Should spring back when touched, skewer test isn't reliable as if there are any big chunks of banana hit it will look like they are under cooked.
Recipe courtesy of Lisa, United Cakedom