Preheat oven to 250. Grease 9x5 inch loaf pan. In an electric mixer, cream butter amd sugar until light and fluffy. Add eggs and beat to incorporate.
In a medium bowl, whisk together flour, baking soda and salt. Add to butter mixture, and mix until just combined. Add bananas, sour cream and vanilla, mix to combine and add the nuts, and pour into prepared pan.
Bake until a toothpick inserted into the center of the bread and it comes out clean, about 1 hour and 10 minutes. Then turn out onto a rack to cool.