Heat oven to 350°. Add butter, peanut butter and water in a saucepan; bring to a boil. Add flour and sugar to a large mixing bowl. Add boiled ingredients; mix well. Beat in eggs, buttermilk, vanilla and baking soda. Pour in a greased baking pan. Bake for 18 to 20 minutes or until toothpick inserted in center of cake comes out clean. Allow cake to cool. Icing: Add butter, peanut butter and milk to saucepan. Bring to a rolling boil and remove from heat. Add milk as needed to reach spreading consistency. Spread quickly; frosting sets up like peanut butter fudge. Yields 25 servings.