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Preheat oven to 350°F.
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In a large mixing bowl, beat together butter, 1 cup sugar, and egg.
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Combine the flour, baking powder, baking soda, and salt.
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Add the flour mixture to the egg mixture alternately with the buttermilk.
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In a small bowl, combine the cinnamon and remaining 1/3 cup sugar.
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Pour a third of the batter into a greased 8X4X2-inch loaf pan.
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Sprinkle a third of the cinnamon sugar on top of batter.
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Repeat layers twice.
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Bake for 45-50 minutes, until a toothpick inserted near the center comes out clean.
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Cool in pan for 10 minutes.
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Remove from pan to a wire rack to cool completely.