Preheat oven to 350° and line two 8″ cake pans with parchment paper and grease with cooking spray. In a large bowl, cream together butter and sugars until smooth. Add eggs, one at a time, beating well after each one, then add vanilla. Add bananas and sour cream and beat until smooth.
In a medium bowl, whisk together flour, cornstarch, baking soda, cinnamon, and salt. Add dry ingredients to wet ingredients and beat until just combined.
Divide evenly between prepared pans and bake until tops are slightly golden and a toothpick inserted into the center comes out clean, 30 to 35 minutes.
Let cool 15 minutes, then invert onto cooling rack to cool completely.
In a large bowl with a hand mixer, beat together cream cheese and butter until smooth. Add powdered sugar and beat again until smooth then add vanilla and heavy cream and beat until a spreadable consistency.
Spread about half the frosting onto one layer, top with second layer, then frost top. Garnish with banana and walnuts.