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Beat butter at medium speed with an electric mixer about 2 minutes or until soft and creamy.
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Gradually add sugars, beating at medium speed 5 to 7 minutes.
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Add eggs, one at a time, beating until yellow disappears.
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Combine flour, baking soda, and cocoa; add to creamed mixture, alternately with sour cream and ending with flour mixture.
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Mix at lowest speed just until blended after each addition.
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Stir in vanilla.
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Spoon batter into a greased and floured 10-inch tube pan.
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Bake at 325°F for 1 hour and 20 minutes or until a wooden pick inserted in center comes out clean.
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Cool pan on a wire rack 10 to 15 minutes; remove from pan, and cool completely on a wire rack.
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Sprinkle with powdered sugar, if desired.