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1
Whisk sugar, 1/2 cup cream, and yolks in medium saucepan to blend. Place over medium heat and stir until sugar dissolves and sauce thickens and reaches temperature of 180u00b0F, scraping down sides of pan occasionally, about 6 minutes (do not boil). Transfer to medium bowl. Refrigerate brown sugar mixture until cold and very thick, stirring occasionally, at least 3 hours and up to 1 day.
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2
Line 7 1/2x3 1/2x2 1/4-inch loaf pan with plastic wrap, leaving overhang. Using electric mixer, beat butter in large bowl until fluffy. Add peanut butter; beat until smooth. Beat in brown sugar mixture in 3 additions. Beat remaining 1/2 cup cream in medium bowl until peaks form. Fold whipped cream into peanut butter mixture in 2 additions. Transfer mousse to prepared pan. Refrigerate while preparing glaze.
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3
Bring cream, butter, and corn syrup to simmer in heavy medium saucepan, stirring often. Remove from heat. Add chocolate; whisk until melted and smooth. Transfer 1/3 cup glaze to small bowl and cool to room temperature; spread over mousse in pan. Cover loaf pan and freeze mousse overnight. Cover and chill remaining glaze.
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4
Place rack on rimmed baking sheet. Turn mousse out onto rack, chocolate side down. Peel off plastic wrap. Transfer 1 1/3 cups glaze to small saucepan. Stir over low heat just until warm and pourable but not hot. Gradually pour glaze over mousse, spreading to cover evenly and using any glaze that drips onto baking sheet if needed. Press chopped peanuts onto top of mousse. Transfer to platter and freeze until glaze is frozen, about 30 minutes.
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5
(Can be made 2 days ahead. Cover loosely when frozen solid and return to freezer.)
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6
Rewarm remaining glaze to use as sauce. Dip large knife into hot water to warm blade. Cut mousse crosswise into thin slices, rewarming and wiping blade dry before cutting each slice. Place mousse on plates. Spoon some sauce alongside and serve.
