Heat oven to 325°. In heavy 1 1/2 qt. saucepan, heat butter, whipping cream and brown sugar over low heat, stirring occasionally, just until butter is melted. Pour into 2 ungreased 9" round cake pans. Sprinkle evenly with peanuts.
Make cake batter as directed on box. Carefully spoon batter into pans.
Bake 35 to 40 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 (I usually place on plates instead), placing peanut sides up. Cool completely, about 1 hr.
Gently stir candy pieces into frosting. Place 1 cake layer, peanut side up, on a serving plate. Spread with half of the frosting mixture. Top with second layer. Spread top with remaining frosting mixture. Cover and refrigerate at least 2 hrs, but not longer than 24 hrs.