For the Cake:
For the Frosting:
Preheat the oven to 350 degrees Fahrenheit. Butter and flour a 9x2-inch round cake pan.
In the bowl of an electric mixer fitted with the paddle attachment, mash the bananas on low speed. Add in the sugar and mix until combined. With the mixer still on low, add in the oil, eggs, sour cream, vanilla and orange zest.
In a separate bowl, sift together the flour, baking soda and salt. Gradually add to the mixer with the speed still on low until the flour and banana mixture is just combined. Fold in the chopped walnuts with a rubber spatula and then turn the batter out into the prepared cake pan.
Bake for 45-50 minutes or until a toothpick inserted in the center comes out clean. Cool the cake in its pan for 15 minutes and then turn out on a wire rack to cool further.
In the meantime, make the frosting. Mix the cream cheese, butter and vanilla together in a large bowl with an electric hand mixer until just combined. Do not whip the mixture. Slowly add the sifted powdered sugar and mix until smooth.
Spread the frosting thickly (obviously) on top of the cake and sprinkle with the chopped walnuts.