Grease an 10-inch round springform pan and line the bottom with a round of parchment paper. (Alternately, you can bake this as a 9x13 cake. It will be a little shorter and will bake in the 60-65 minute range. You may want to up the frosting amounts to 8 oz. cream cheese/4 Tbsp butter/ 2 tsp vanilla and 3 cups icing sugar, if you want a nice thick layer of frosting).
In a large bowl with an electric mixer or in the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar. Add the sour cream and mashed bananas and mix until combined. Beat in the eggs one at a time, beating well after each addition and scraping down the side of the bowl, as needed. Stir in vanilla.
In a medium bowl, whisk together the flour, baking soda, baking powder and salt until thoroughly mixed.
Add the flour mixture to the batter in three additions, alternating with the milk in between flour additions, until all is added and just combined. Pour batter into prepared pan.
Bake cake in preheated oven for anywhere from 1 hour and 20 minutes - 1 hour and 45 minutes, or until a toothpick inserted into the center of the cake comes out clean (test and be sure!). Remove from oven and allow to stand for 10 minutes, then cover the top with aluminum foil and place in to the refrigerator for at least 1 hour, or until completely cool.
For the frosting: In a medium bowl with an electric mixer or the bowl of a stand mixer with a paddle attachment, beat together the cream cheese and butter. Add the vanilla and icing sugar and beat until smooth and glossy.
Remove cake from refrigerator and remove the foil from the top. Spoon the prepared frosting on top of the cake and spread completely over the top in an even layer. Sprinkle top with finely chopped walnuts, if using. Gently run a knife around the edge of the pan, then loosen the outer ring of the springform pan. Carefully run a knife around under the cake, between the ring bottom and the parchment paper. When loosened, slide the cake, still with the parchment on the bottom, onto a serving platter.