Prep. Preheat the oven to 350°F and grease an 8X8-inch baking dish liberally with baking spray or butter.
Cream. In the bowl of an electric mixer, cream together the butter and sugars on high speed until fluffy and noticeably lighter in color (about 2-3 minutes).
Add the remaining wet ingredients. Reduce the mixer to medium speed and mix in the sour cream. Mix in the eggs, one at a time, followed by the vanilla.
Combine the dry ingredients. In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon.
Put it all together. On low speed, gradually mix the dry ingredients into the wet ingredients. Finally, mix in the bananas and fold in the walnuts.
Bake. Spread the batter evenly into the prepared baking dish and bake for 25 minutes or until a toothpick inserted into the center comes out clean.
Cool. Remove the brownies from the oven and allow them to cool completely. In the meantime, make the frosting.
Make the cream cheese frosting. Beat the cream cheese and butter until smooth and creamy. Add the powdered sugar, one cup at a time, beating until smooth after each addition. Stir in the vanilla extract.
Frost the brownies. Once the brownies are completely cool, spread the cream cheese frosting evenly over the top.