Preheat oven to 375 degrees Fahrenheit. Prepare muffin tin with paper liners or baking spray.
In mixing bowl, combine mashed banana, melted margarine, sugar, cornstarch, water, vanilla, and salt. Whisk together until smooth. Then, gently and quickly whisk in your flours and baking powder until everything is well-incorporated.
Prepare crumble topping by mixing chopped walnuts, sugar crystals, and brown sugar in a bowl. Set aside.
Fill oiled and/or lined muffin tins 3/4 full with muffin batter. Sprinkle muffin tops with crumble topping.
Bake at 375 degrees Fahrenheit for about 22-24 minutes, or until muffins are golden brown, and a toothpick comes out clean.