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Preheat oven to 350° Spray a round 9" cake pan with cooking spray; set aside.
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In a large bowl, whisk together flour, granulated sugar, baking powder, optional salt; set aside.
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In a separate small bowl, whisk together the egg, buttermilk, sour cream, oil, and vanilla.
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Add the wet mixture to the dry, mixing lightly with a spoon or folding with a spatula until just combined.
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Turn batter out into prepared pan and bake for about 29 to 32 minutes, or until center is set and not jiggly, and a toothpick inserted in the center comes out clean or with a few moist crumbs, but no batter. Cool for at least 30 minutes before removing from the pan and frosting.