Combine flour, sugar, salt and baking soda in a large bowl; set aside. Combine butter, water and peanut butter in a saucepan over medium heat; bring to a boil. Add to flour mixture and mix well; set aside. Combine eggs, vanilla and buttermilk; add to peanut butter mixture.
Spread batter in a greased 15"x10" jelly-roll pan. Bake at 350 degrees for 25 to 28 minutes or until cake springs back when gently touched. Spread Peanut Butter Icing over warm cake and garnish, if desired.
Peanut Butter Icing: Combine first 3 ingredients in a saucepan over medium heat; bring to a boil. Remove from heat; stir in powdered sugar and vanilla, mixing until spreading consistency.