1 1/4 cups dark brown sugar
1 cup whipped topping, (unwhipped) divided
3 large egg yolks
6 tablespoons (3/4 stick) margarine
1 1/4 cups creamy peanut butter
Glaze and sauce:
1 1/2 cups whipped topping (unwhipped)
2 tablespoons (1/4 stick) margarine
4 teaspoons light corn syrup
12 ounces bittersweet or semisweet chocolate, chopped
1/2 cup coarsely chopped roasted salted peanuts for garnish
Prepare peanut butter mousse:
Whisk sugar, 1/2 cup topping and yolks in medium saucepan to blend. Place over medium heat and stir until sugar dissolves and sauce thickens, 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.
Line a 7 or 8 inch x2 -inch loaf pan with plastic wrap, leaving overhang.
Using electric mixer, beat margarine in large bowl until fluffy. Add peanut butter; beat until smooth. Beat in brown sugar mixture in 3 additions. Beat remaining 1/2 cup topping 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.
For glaze and sauce:
Bring topping, margarine, and corn syrup to simmer in heavy medium saucepan, stirring often. Remove from heat. Add chocolate; whisk until melted and smooth.
Remove 1/3 cup glaze to a small bowl and cool to room temperature; spread over mousse in pan. Cover loaf pan and freeze mousse overnight.
Chill remaining glaze in an airtight container until needed.
Place a cooling rack on a baking sheet. Turn mousse out of loaf pan onto rack, chocolate side down. Peel off plastic wrap.
Remove glaze from the refrigerator and place 1 1/3 cups glaze in a small saucepan. Stir over low heat just until warm and pourable but not hot. Gradually pour melted 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. Cover loosely when frozen solid and return to freezer.
(Can be made 2 days ahead.)
Reheat remaining glaze to use as sauce. Dip large knife into hot water to warm blade. Cut mousse crosswise into thin slices, warming and wiping blade dry before cutting each slice. Place mousse on plates. Spoon some sauce alongside and serve.