Preheat oven to 325F and oil the bread pan.
Cream butter and sugar in large mixing bowl. Add egg and mashed banana. Stir in sour cream and vanilla extract.
Sift in flour so it is not lumpy. Before mixing together, add baking soda, salt, and cinnamon. Mix the dry ingredients into the wet ingredients. Fold in chopped walnuts. Add to oiled bread pan.
Bake in oven for an hour or until toothpick comes out clean.