2/3 cup unsalted butter (softened)
1 1/2 cups (packed) brown sugar
2 large eggs (separated)
3/4 cup mashed banana (about 2 large)
1 teaspoon pure vanilla extract
1/4 cup sour cream
1 1/2 cups unbleached all-purpose flour
  pinch salt
1 teaspoon baking soda
1 cup toasted chopped pecans or walnuts