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1. Preheat the oven to 350 degrees. Spray a 12 or 15 cup bundt pan with baking spray or butter and flour it, set aside.
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2. In the bowl of a stand mixer, cream butter on medium speed for 2 minutes. Add sugar and vanilla, beat until pale and fluffy (~4 minutes). Add egg yolks and banana, mix until combined.
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3. In a medium mixing bowl, whisk together the flour, baking soda, and salt.
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4. With mixing speed on low, alternate adding the dry ingredients and the sour cream, mix just until combined.
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5. In a small mixing bowl with a hand mixer, whip egg whites until stiff peaks form. Using a spatula, gently fold in the egg whites to the batter.
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6. Pour batter into the prepared pan, smooth out into an even layer. Place in the oven and bake for 45-55 minutes or until a cake tester (or toothpick) inserted into the center comes out clean. Remove from the oven and allow to cool in the pan for 20 minutes before inverting onto a wire rack to cool completely.