Preheat oven to 350°F. Grease and lightly flour two 9-inch round cake pans.
Combine cake mix, water, oil, and eggs in large mixing bowl. Beat at low speed with electric mixer until moistened. Beat at medium speed for 2 minutes. Pour into prepared pans.
Bake 30 to 33 minutes or until toothpick inserted in center comes out clean. Cool in pans 15 minutes; remove from pans, cool completely.
Mix frosting and peanut butter in small bowl until smooth. Fill and frost cake. Sprinkle with peanuts.