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Preheat oven to 350F. Line a 9x5-inch loaf pan with parchment paper.
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Beat together butter and sugar with an electric mixer until light and fluffy. Beat in eggs, sour cream, bananas, and vanilla.
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In a separate bowl. sift together flour, baking powder, baking soda, and salt. Add this mixture to the butter mixture and beat until just combined. Use a rubber spatula to stir in the walnuts.
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Pour batter in the prepared pan.
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Bake for about 1 hour or until a toothpick inserted into the center comes out clean.
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Cool 5 minutes before inverting onto a wire rack.