1. Preheat oven to 325°.
2. Cream butter and sugar in a bowl with an electric mixer on medium speed 1 minute or until well-incorporated. Add banana, yogurt, and sour cream; beat on low speed until combined.
3. Whisk together eggs and egg whites in a separate bowl. Sift oat flour and the next 3 ingredients (through salt) in a large bowl. Add butter and egg mixtures alternately into oat flour mixture.
4. Add vanilla, and pour batter into 10-inch tube pan coated with cooking spray. Bake 1 hour or until a wooden pick inserted in cake comes out clean. Serve toasted, and topped with 1/4 cup berries and 1/2 teaspoon powdered sugar.