Preheat oven, prepare pan, and cream butter and sugar: Preheat oven to 350 degrees. Butter a 9-by-5-by-3-inch loaf pan; set aside. In an electric mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy.
Add eggs: Add eggs and beat to incorporate.
Whisk dry ingredients and add to butter mixture: In a medium bowl, whisk together flour, baking soda, and salt. Add to the butter mixture and mix until just combined.
Add bananas, sour cream, and vanilla: Add bananas, sour cream, and vanilla; mix to combine.
Add nuts and transfer to prepared pan: Stir in nuts and pour into prepared pan.
Bake: Bake until a cake tester inserted into the center of the cake comes out clean, about 1 hour 10 minutes.
Cool: Let rest in pan for 10 minutes, then turn out onto a rack to cool.
