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In small bowl, pour boiling water over chocolate; let stand 20 minutes to cool.
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Meanwhile, preheat oven to 325 degrees. Grease well and flour 9x5x3-inch loaf pan.
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Sift flour with soda and salt.
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In large bowl of electric mixer, let butter stand at room temperature until softened. At high speed, beat butter, brown sugar, eggs, and vanilla until light and fluffy.
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At low speed, beat in flour mixture, in fourths alternately with sour cream, in thirds, beginning and ending with flour mixture.
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Beat in cooled chocolate mixture just until combined.
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Pour into prepared pan.
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Make 60 to 70 minutes, until cake tester comes out clean.
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Cool in pan on wire rack 15 minutes. Transfer from pan to rack; cool completely.