PREHEAT oven to 350 degrees. Grease and flour a 9 x 2-inch round cake pan, or line the pan with a piece of parchment or wax paper cut to fit inside the cake pan. Spray the liner and sides lightly with cooking spray.
MIX bananas, granulated sugar and brown sugar in bowl with an electric mixer on low speed until combined. Add oil, eggs, sour cream and vanilla. Mix until smooth.Add flour, baking soda and salt. With mixer on low, mix just until combined. Stir in chopped walnuts if using. Pour batter into the prepared pan and bake for 40-45 minutes, until a toothpick inserted in the center comes out clean. Cool in pan for 5 minutes, turn out onto cooling rack.
Serve warm, sprinkled with powdered sugar, or cool completely and top with Cream Cheese Frosting.