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Preheat oven to 350.
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Butter and lightly flour a 9x5" loaf pan.
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Grate or chop chocolate until you have slivers and small pieces. Set aside.
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In large bowl with mixer at medium speed, beat butter and sugar until light and fluffy.
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Reduce speed to low; add flour, baking powder, baking soda, vanilla, sour cream and eggs.
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Beat well, scraping down bowl as necessary.
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Spread half of batter evenly into pan, sprinkle with half grated chocolate and 1/4 t. cinnamon.
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Cover with remaining batter, chocolate and cinnamon.
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Draw knife through batter a few times to swirl chocolate.
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Bake 1 hour (or less, be careful) until toothpick comes out clean.
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Cool in pan on wire rack 10 minutes.