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Preheat oven to 325 degrees.
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In a bowl, beat 1 cup butter, 1 cup sugar, cocoa, and chocolate with a mixer until fluffy.
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Add eggs and sour cream.
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Beat until well mixed.
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Mix together baking soda, baking powder, and 2 cups flour.
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Stir into batter, then beat to blend well.
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Butter and flour-dust a 10- to 12-cup tube pan or bundt pan.
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Scrape batter into pan and spread level.
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Bake until cake just begins to pull from pan sides and center springs back when lightly pressed, about 1 hour.
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Meanwhile, in a medium sauce pan over high heat, stir 2 cups sugar with 1 cup water until sugar dissolves.
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Remove syrup from heat and add liqueur.
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Run a knife blade between cake and pan.
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Let cake cool in pan on a rack for about 10 minutes; invert cake onto rack, then tip cake back into pan.
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With a thin skewer, pierce cake all the way to the bottom of pan, making holes an inch apart.
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Slowly pour syrup over cake.
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Let cool about 1 1/2 hours.
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Dip cake pan in hot water almost to rim; wipe pan dry.
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Invert cake onto a plate.