Preheat oven to 350 degrees.
Spray a 9x13 baking pan with non-stick cooking spray.
In a mixing bowl, cream together butter, egg, sugar and mashed bananas.
Mix in All-Bran Cereal and then melted chocolate.
Sift flour, baking powder, baking soda, salt, and cinnamon into the wet mixture.
Add milk. Mix to combine.
Fold in walnuts.
Spread into prepared baking dish.
Bake for 30 minutes, or until a toothpick inserted in the center comes out clean.
Cool in pan.
To prepare the frosting: Melt butter and chocolate chips together, stir in powdered sugar, milk, salt and vanilla.
Spread frosting on cooled bars.
Cut into 32 squares.