Thoroughly grease a 12-cup Bundt cake pan with nonstick cooking spray.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar until fluffy.
Add the eggs, mashed bananas, sour cream, and vanilla extract and beat until smooth.
Beat in the flour, salt, and baking powder until combined, make sure you don't overbeat the batter. Set aside.
In a small bowl combine the brown sugar, flour, cinnamon, nutmeg, and melted butter until well combined and smooth.
Spread ⅓ of the batter evenly into the prepared Bundt pan.
Add half of the cinnamon swirl mix to the batter and use a fork or butter knife to swirl the mixture throughout the batter. Repeat with the remaining ⅓ batter and ½ cinnamon swirl mixture, topping everything with the remaining ⅓ batter.
Bake in the preheated oven for 80-90 minutes, until a toothpick inserted into the cake comes out clean.
Let the cake cool for 10 minutes in the pan, then invert it onto a cooling rack and let cool completely.
Meanwhile, beat the cream cheese and butter until smooth.
Add in the confectioners' sugar, milk, and vanilla extract and beat until combined.
Drizzle the icing over the cooled cake. Serve at room temperature.