Preheat the oven to 350°F (177°C). Adjust the oven rack to the lower 1/3rd position so prevent the top of the bread from over-browning.
Place the bananas in a mixing bowl. Mash bananas with a fork or potato masher. Set aside.
In a medium bowl, combine the all purpose flour, baking soda, salt, and ground cinnamon. Set aside.
In the bowl of a stand mixer, add the butter and brown sugar. Beat on medium-high speed for three minutes until smooth.
Add the eggs one at a time into the stand mixer bowl. Beat on medium speed until each egg is incorporated into the mixture. Then, add the sour cream, mashed bananas, and the vanilla extract. Mix on medium speed until combined.
Add the dry ingredients. Add the flour mixture into the stand mixer bowl. Beat on low speed until all of the ingredients are combined and there are no more dry flour bits.
OPTIONAL Fold the chopped walnuts into the batter with a silicon spatula.
Grease an 8x8-inch square pan or cake pan. Pour the batter into the pan until it is 75% full. (Check notes for instructions on using different-sized pans).
Bake for 55-65 minutes. At the 30-minute mark, cover the banana bread with loose aluminum foil to prevent the top from over-browning. (Check notes for bake time)
Notes
Check my post for detailed step-by-step photos of the entire recipe.
Batter Level For Different-Sized Pans: The type of pan you use changes how much batter to put into the pan. I used an 8x8-inch square pan and filled it ½ of the way. For a cake pan, fill ½ of the way. For a muffin pan, fill to the top.
Can I use a Cake Bundt Pan? There isn't enough batter to make a bundt cake. However, if you only have a bundt pan, then go for it. Just know that you will not get a cake.
Baking Time: Baking time is dependent on the type of pan that you use and your oven. For an 8x8-inch square pan, it takes roughly one hour. If you are using a muffin pan, then it will take roughly 45 minutes. Make sure to use the toothpick test that your banana bread is done.
Toothpick Test: your banana bread is ready when you insert a toothpick into the center and there are a few crumbs on it. If the toothpick has wet batter, it still needs to bake for longer or else you'll end up with a sunken middle. Make sure not to bake for longer than needed or else you won't have moist banana bread.
Storing: Your banana bread will last at room temperature for up to four days. If you need to store it for longer, you can put it in the refrigerator or freezer.