Preheat oven to 350F degrees. Mix cake mix according to box. Pour into 2 round cake pans. Put in oven and bake according to package. While cake is baking, mix box pudding with 1 cup of cold milk until it thickens. Add 1/2 cup peanut butter to pudding mixture and stir until completely mixed. Put mixture in refrigerator. When cakes are finished baking and cooled on wire racks, take around 3/4 cup of pudding mixture and spread it on bottom cake layer. Place top cake layer over filling, then take 1 1/2 cups cool whip and fold it in the remaining pudding mixture until mixed well. Then frost the outside of your cake.