Preheat oven to 350 degrees. Grease n flour 10" bundt pan or 13"x9" pan.
In large bowl stir together cake mix n pudding mix. Make a well in center and pour in eggs,water,oil n mashed banana. Beat on low spead until blended. Scrape down bowl and beat 4 minutes on medium speed. Pour into prepared pan. Bake in preheated oven for 50-55 minutes or utill test done.Cool 10 minutes n turn out on rack to cool completely if using a bundt pan, if using 13X9 pan just cool completely in the pan.
To make the glaze in small bowl combine powdered sugar milk n vanilla,Whisk till smooth and of drizzling consistency. When cake is cooled drizzle glaze over cake in ziz zag pattern. Sprinkle walnuts over wet icing.
I have also added walnuts or pecan to cake itself.