Line 8-inch square pan with heavy-duty foil, leaving extra foil extending over edges. Spray with nonstick cooking spray.
Place 3/4 cup butter and chocolate in medium heatproof bowl; place over medium saucepan of barely simmering water. Stir frequently until mixture is melted and smooth. Remove bowl from saucepan; let stand until cooled slightly.
In large bowl, beat eggs, sugar and salt at medium speed until blended. At low speed, beat in chocolate mixture and 1 teaspoon vanilla until blended. Beat in flour just until incorporated. Gently stir in peanut butter cups. Spread batter in pan.
Bake 40 to 50 minutes or until toothpick inserted in center comes out with moist crumbs attached (be careful not to hit peanut butter cups). Top may crack during baking. Cool completely on wire rack. (Bars may sink slightly during cooling.).
Meanwhile, in another large bowl, beat cream cheese and 1/4 cup butter at medium speed until blended and smooth. Beat in 1 teaspoon vanilla. At low speed, beat in 2 1/2 cups of the powdered sugar until blended and smooth, adding additional powdered sugar, if necessary, to achieve desired spreading consistency. Stir in peanut butter. Spread frosting over bars; sprinkle with peanuts.
Using foil edges, lift bars from pan. Slide bars off foil onto cutting board; cut into 16 pieces.
(Bars can be made up to 2 days ahead. Cover and refrigerate. Bring to room temperature before serving.).