Preheat oven to 375°F. Lightly spray mini muffin pan with vegetable pan spray.
In large bowl, combine the flour, baking powder, baking soda and salt; set aside. In medium bowl, whisk together the egg, brown sugar, milk, oil, vanilla, bananas and walnuts. Stir into flour mixture just until combined. Divide mixture into prepared pans, filling almost full. Sprinkle muffins with additional walnuts, if desired.
Bake 12 to 14 minutes or until firm to the touch. Cool muffins in pan 5 minutes. Transfer to wire rack to cool completely.