Preheat oven to 350°F. Beat cake mix, eggs, bananas, sour cream and oil with mixer on low until moistened. Scraping bottom of bowl often. Beat on medium 2 min. Pour into greased and floured 13 x 9 inch pan. Bake 35 min or until toothpick inserted in center comes our clean. Cool cake completely in pan on wire rack.
Beat cream cheese and butter with mixer on medium until blended. Gradually add sugar, beating well after each addition.
Remove cake from pan. Carefully cut cake crosswise in half using serrated knife. Place 1 cake half, top side down, on plate; spread top with frosting. Top with remaining cake half, top-side up. Frost top and sides. Press nuts onto sides. Refrigerate leftovers.