Preheat oven to 325ºF (163ºC). Spray a 12-cup bundt pan with nonstick cooking spray and add a few tablespoons of all-purpose flour to the pan, shake it around to coat well, then tap out and discard any excess.
In the bowl of an electric stand-up mixer, beat butter (1 1/2 cups) until it's creamy. Add sugar (3 cups) and beat at medium speed for 5-7 minutes, or until the mixture is light and fluffy. (the exact time will depend on how soft the butter was when you started). Add eggs, one at a time, beating just until the yolk disappears and is incorporated into the batter.
In a medium bowl, mix flour (3 cups), salt (1/2 teaspoon), and baking soda (1/2 teaspoon). Add to butter mixture, alternating with sour cream, beginning and ending with the flour mixture. Beat batter on low just until blended after each addition.
With the mixer on low, stir in vanilla (1 tablespoon) and banana mash (2 cups) and continue to beat for about 1 minute. Using a large rubber spatula or wooden spoon, give the batter a final stir and make sure the mixture is well combined.
Pour the batter into the prepared pan, filling the pan only 3/4 full, and bake in the preheated oven for 1 hour and 30 minutes to 1 hour and 50 minutes, testing until a wooden skewer or cake tester, inserted into the center of the cake just comes out clean, with just a few crumbs.
Cool the pan on a wire rack for 10-15 minutes, then invert the bundt pan onto a plate, remove the cake from the pan, and cool completely.
Prepare the glaze by mixing cream cheese (4 ounces), powdered sugar (2 cups), and vanilla (1 teaspoon) in a medium bowl with a handheld mixer. Add milk 1 tablespoon at a time to get a thick pourable consistency. Place the glaze in a 1-quart resealable bag. Snip off one corner. Squeeze glaze over completely cooled cake. Allow glaze to set and serve.