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Preheat oven to 375F. Line a 9x9 square pan with parchment (or butter and flour).
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Cream together butter and sugar until light in color and fluffy. Add in sour cream, mashed banana, egg, and vanilla. Beat until combined.
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In a separate bowl, sift together flour, salt, and baking soda.
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Add dry ingredients to wet ingredients and beat until just combined.
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Pour mixture into the prepared pan. Bake in preheated oven for 22-26 minutes, or until a toothpick inserted into the center comes out clean.
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Let cool completely before frosting.