Heat oven to 177°C (163°C for dark or nonstick pans). Generously grease bottoms only of 2 (20- or 23-cm) round cake pans with shortening or cooking spray.
In large bowl, beat cake mix, water, 1/2 cup peanut butter, the oil and eggs with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour into pans.
Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes. Run knife around sides of pans to loosen cakes; remove from pans to cooling rack. Cool completely, about 1 hour.
In 1.9 L saucepan, melt butter over medium heat; stir in brown sugar. Heat to boiling; boil and stir 1 minute. Remove from heat. Refrigerate 10 minutes.
In chilled medium bowl, beat whipping cream on high speed until soft peaks form; set aside. In another medium 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; beat on medium speed until mixture is smooth and creamy.
Split each cake layer horizontally to make 2 layers. Fill each layer with about 2/3 cup peanut butter mixture to within 1 cm of edge. Frost side and top of cake with frosting. Press peanuts onto frosting on side of cake. Store covered in refrigerator.