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Preheat the oven to 350°. Spray a 9- x 5-inch loaf pan with nonstick cooking spray.
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In a large bowl, stir together the flour, sugar, baking powder, baking soda, and salt.
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In a separate bowl, stir together the egg whites, oil, buttermilk, and vanilla.
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Make a well in the center of the flour mixture and stir in the buttermilk mixture until evenly moistened.
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Scoop out 2/3 cup of the batter and transfer it to a medium bowl. Add the chocolate syrup and cinnamon and stir until combined.
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Spoon the remaining batter into the loaf pan. Drop the chocolate batter onto the vanilla batter and, with a knife, swirl the chocolate batter through the vanilla batter.
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Bake for 45 to 50 minutes, or until a toothpick inserted in the center comes out clean. Cool the loaf in the pan for 10 minutes, then invert onto the rack to cool completely. Makes 10 slices.