Preheat the oven to 350°F. Lightly grease (2) 8" x 4” loaf pans or 12-18 muffin cups. Combine the butter and sugar in a mixing bowl and beat with a mixer until smooth. Add the vanilla and eggs and beat again until smooth. Slowly add the dry ingredients, beating just until combined.
Stir in the bananas and sour cream and beat just until combined. Pour into the prepared pans or muffin tins. Bake the muffins for 18-20 minutes and bake the loaves for 55-60 minutes, until an inserted toothpick comes out with moist crumbs.