Grease or spray bottoms only of two 8 or 9 inch pans.
In large bowl, beat cake mix, water 1/2 cup peanut butter, and eggs on low speed for 30 seconds, then on medium speed for 2 minutes, scraping bowl often.
Pour into pans.
Bake 31 to 36 minutes. Cool 10 minutes. Run knife around sides of pan to loosen cake, then place cake on cooling rack. Cool compleately, about 1 hour.
In a 2 qt. sacepan, melt 1/4 cup butter over medium heat, stir in brown sugar. Heat to boiling, boil and stir 30 seconds. Remove from heat. Refrigerate uncovered for 10 minutes.
In a chilled bowl, beat whipping cream on high speed until soft peaks form; set aside. In another bowl, beat 1/2 cup peanut butter and the brown sugar mixture on medium speed until smooth and creamy. Add whipped cream to peanut butter mixture, and beat until smooth.
Split each cake layer horizontally. Fill each layer with about 2/3 cups peanut butter mixture to within 1/2 of cake. Put cake layers together.
Frost cake with chocolate frosting. If desired, press chopped peanuts onto sides and top of cake.