Adjust oven rack to lower-middle position and heat oven to 350°F. Spray an 8x8-inch baking pan with nonstick spray and line it with a parchment paper sling (with a 2-inch overhang on each side). Set aside.
Whisk the flour, baking soda, salt, and cinnamon together in a medium bowl. Set aside.
1½ cups all-purpose flour, 1 teaspoon baking soda, 2 teaspoons kosher salt, 1 teaspoon ground cinnamon
In a separate medium bowl, mix the bananas, sour cream, and vanilla together. Set aside.
3 very ripe bananas, ½ cup sour cream, 1 tablespoon pure vanilla extract
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until creamy, about 3 minutes.
8 tablespoons unsalted butter
Add the granulated sugar and brown sugar and continue beating until light and fluffy, about 3 minutes. Scrape down the sides of the bowl, as needed.
½ cup granulated sugar, ½ cup brown sugar
Add the eggs and mix until incorporated.
2 large eggs
Add the flour mixture and mix on low speed until just a little flour still shows.
Add the banana mixture and mix, on low speed, until combined.
Remove the bowl from the mixer and scrape down the sides with a rubber spatula. Finish mixing until all ingredients are incorporated.
Pour the cake batter into the prepared baking dish and bake at 350°F for 45-55, minutes or until a toothpick inserted in the center comes out clean. The top will be a dark golden brown.
Transfer the cake to a cooling rack and cool 20 minutes. Use the parchment sling to carefully remove the cake from the pan. Peel the sides of the parchment paper away from the cake and cool completely.