Savor the comforting flavors of this moist and nutty banana walnut cake. Mashed ripe bananas and toasted walnuts create a cake that is rich in flavor and texture. The secret to its moistness lies in the addition of sour cream, which keeps the cake incredibly tender. Topped with a creamy cream cheese frosting and sprinkled with toasted walnuts, this cake is a delicious way to use up overripe bananas.