Preheat oven to 350 degrees. Place 2 pieces of parchment paper in a 13X9-inch pan perpendicular to each other and spray with cooking spray. Make sure parchment paper extends over edges of pan so you can use the parchment paper to lift the cake out of the pan.
In a large bowl, whisk together the flour, baking soda, and salt. Set aside.
Using an electric mixer fitted with a paddle attachment, beat butter and sugars at medium speed until light and fluffy, about 3 minutes. Add eggs one at a time. Turn mixer off and scrape sides of bowl with a rubber spatula.
With mixer on low-speed, add sour cream and vanilla. Add banana and once incorporated, remove bowl from mixer and hand stir in flour and chocolate chips.
Pour batter into prepared pan and bake for approximately 25 minutes or until toothpick inserted in middle comes out with just a few moist crumbs attached. Transfer pan to wire rack to cool.
For frosting, using an electric mixer with a paddle attachment beat butter and cream cheese until smooth.
Gradually add powdered sugar and then vanilla extract.
To make walnuts, place walnuts and sugar in a small heavy-bottomed saucepan over medium heat. Stir constantly. After a few minutes, the sugar will begin to liquefy and coat the walnuts. Be careful not to let the mixture burn at this point. Keep stirring until the walnuts are well coated. Remove from heat and spread on a piece of parchment paper to cool.
Remove cake from pan by lifting the edges of the parchment paper. Spread cake with frosting and top with sugared walnuts.