Heat oven to 163°C. In heavy 1.4L saucepan, heat butter, whipping cream and brown sugar over low heat, stirring occasionally, just until butter is melted. Pour into 2 ungreased 23 cm round cake pans. Sprinkle evenly with peanuts.
Make cake batter as directed on box. Carefully spoon batter into pans.
Bake 36 to 41 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, placing cakes peanut sides up. Cool completely, about 1 hour.
Gently stir candy into frosting. Place 1 cake layer, peanut side up, on serving plate. Spread with half of the frosting mixture. Top with second layer, peanut side up. Spread top with remaining frosting mixture. Cover and refrigerate at least 2 hours but no longer than 24 hours.