Prepare cake mix according to package directions. Pour into 2 lightly greased 8-inch round cake pans. Bake according to package directions. Cool in pans on wire racks 10 minutes. Remove from pans and cool completely.
Combine 2/3 cup of whipping cream and peanut butter mosels 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 of whipping cream at high speed with an electric mixer until soft peaks form. Stir 1/3 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 with peanuts. Serve warm or chill and serve at room temperature.
ALTERNATELY: You can make this into parfaitsby cutting cooled layers into cubes and layer 8 parfait glasses evenly with 2/3 peanut butter mixture, cake cubes, and half of the chocolate mixture. Ending parfaits with remaining peanut butter mixture and chocolate mixture; sprinkle each parfait with chopped peanuts, if desired.