Prepare cake mix according to package directions. Pour into 2 lightly greased 8-inch round cakepans. Bake according to package directions. Cool in pans on wire racks 10 minutes. Remove from pans, and cool completely.
Combine 2/3 cup whipping cream and peanut butter morsels in a small saucepan. Cook over low heat, stirring constantly, until morsels melt and mixture is smooth; remove saucepan from heat. Stir in vanilla extract, and cool 25 minutes.
Beat 1 1/3 cups whipping cream at high speed with an electric mixer until soft peaks form. Stir one-third whipped cream into peanut butter mixture. Fold into remaining whipped cream. Spread between cake layers; cover and chill.
Bring remaining 1/2 cup whipping cream and 1 tablespoon butter to a boil in a small saucepan. Add chocolate morsels; remove from heat, and let stand 5 minutes. Stir until morsels melt and mixture is smooth.
Pour glaze over cake; garnish, if desired. Serve warm, or chill and serve at room temperature.
Chocolate-Peanut Butter Mousse Parfaits: Cut cooled cake layers into cubes. Layer 8 Parfait glasses evnly with two-thirds peanut butter mixture, cake cubes, and half of cocolate mixture. Top with remaining peanut butter mixture and chocolate mixture; sprinkle each parfait with chopped peanuts, if desired.