Preheat over to 350°. Grease a loaf pan and set aside.

In a large bowl, stir together the butter and sugar until fluffy. Add the eggs and vanilla, mixing well. Combine the flour, baking soda, and cinnamon and stir into the butter/sugar mixture until smooth. Finally, fold in the sour cream and mashed bananas. Spread evenly into prepared loaf pan.

Bake at 350° for 60 minutes, or until a toothpick inserted into several center spots comes out clean. Store covered.