Preheat oven to 325°F (165°C). Grease two 9x5-inch loaf pans with butter or cooking spray, line bottoms with parchment paper, and coat the bottom and sides of each pan with about 2 tablespoons of the cinnamon sugar topping mixture. Reserve the remaining topping.
Peel the ripe bananas and mash thoroughly with a fork until mostly smooth with just a few small lumps — you should have about 1½ cups. Set aside.
In a large mixing bowl, beat the softened butter and 2 cups of granulated sugar together on medium speed for 3–4 minutes until light, fluffy, and pale. Scrape down the sides of the bowl as needed.
Add eggs one at a time, beating well for about 30 seconds after each addition, then mix in the vanilla extract until fully incorporated.
Mix in the mashed bananas on low speed until just combined, then add the sour cream and mix until no white streaks remain — do not overmix at this stage.
In a separate medium bowl, whisk together the flour, baking soda, and salt until well combined and lump-free.
Add the dry ingredients to the wet mixture in three additions, mixing on low speed after each just until no dry flour streaks remain — overmixing will make the bread tough.
Divide the batter evenly between the two prepared loaf pans, filling each about two-thirds full, smooth the tops, and sprinkle the remaining cinnamon sugar evenly over each loaf.
Bake on the center rack for 1 hour 15 minutes to 1 hour 30 minutes, until a toothpick inserted in the center comes out clean and the tops are deep golden brown — tent loosely with foil during the last 20 minutes if browning too quickly.
Cool in pans on a rack for 15–20 minutes, then turn out, peel off parchment, and allow to cool completely for about 1 hour before slicing with a serrated knife.