Heat oven to 350ºF. Grease a 9x13-inch pan on the bottom and sides by rubbing them with a cold stick of butter.
Beat cake mix, eggs, bananas, sour cream, and oil with a mixer on low until moistened, scraping the bottom of the bowl often. Beat on medium 2 minutes.
Pour cake batter into the parepared pan. Bake 35 minutes or until a toothpick inserted in the center comes out clean. Cool cake completely in the pan on a wire rack.
Beat cream cheese and butter with mixer on medium until blended. Gradually add sugar, beating well after each addition.
Remove the cake from the pan. Carefully cut the cake horizontally in half using a serrated knife. Place one cake half, top-side down, on a platter; spread with frosting. Top with the remaining cake half, top-side up. Frost the top and sides of the cake.
Press chopped walnuts onto the sides and sprinkle over the top.
Refrigerate leftovers.
Notes
Refrigerate leftovers in an airtight container for up to 5 days.