For the layer cake: In a large mixing bowl, beat cake mix, water, peanut butter, oil and eggs with electric mixer on medium speed about 2 minutes. Pour into greased and floured 9 inch round pans.
Bake the cake for 30 minutes at 350 degrees or until a toothpick inserted in the center comes out clean. Cool for 10 minutes, then invert the round cakes onto a cooling rack to cool completely.
For the Peanut Butter Frosting: In a mixing bowl, blend together peanut butter, powdered sugar, butter, and milk with an electric mixer on medium speed for about 3-3minutes or until light and fluffy.
Level the cakes if necessary, then place one cake on a plate and frost the top. Place the other cake on top of that and frost the sides and top. Decorate as desired with piping rosettes and using Nutter Butters, large or bite-sized.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.