To make a moist banana cake, use ripe bananas, vegetable oil, large eggs, sour cream and other ingredients. Make a tangy frosting with cream cheese, unsalted butter, pure vanilla extract and sifted confectioners' sugar.
Preheat oven to 350 degrees Fahrenheit, and coat a 9-by-2-inch round pan with butter and flour. Combine 3 mashed bananas, 3/4 cup granulated sugar and 1/2 cup light brown sugar. Stir with an electric mixer on low speed. Include 1/2 cup vegetable oil, 2 eggs, 1/2 cup sour cream, 1 teaspoon vanilla extract and grated orange zest, and mix until even.
Place 2 cups flour, 1 teaspoon baking soda and 1/2 teaspoon salt in another bowl, and stir on low speed. Add the mixture to the batter along with 1/2 cup walnuts, if desired. Pour the batter into the pan, and cook for 45 to 50 minutes Let the cake cool in the pan for approximately 15 minutes, and flip the finished cake onto a cooking rack.
For the frosting, mix 6 ounces cream cheese, 6 tablespoons butter and 1 teaspoon vanilla extract with an electric mixer on low speed. Gradually stir in 2 1/2 cups confectioners' sugar. Frost the banana cake, and decorate with walnut halves.