CAKE: Preheat oven to 400 degrees. Grease and lightly flour a 10×15×1-inch jelly roll pan or baking sheet.
In a large bowl, mix together flour, sugar, baking soda and salt, set aside.
Combine peanut butter, butter and water in saucepan, bring to a boil, remove from heat. Pour over flour mixture, beat to combine. Add milk, vanilla and eggs, mix well.
Pour batter into prepared baking sheet. Bake for 20-25 minutes or until cake tests done with a toothpick. Cool on a wire rack, in the pan.
FROSTING: Bring peanut butter, butter and milk to boil in a saucepan, stirring constantly, remove from heat. Add one pound of confectioners' sugar, mix well. Pour warmed frosting over cooled cake. Sprinkle with chopped peanuts if desired. Serve from pan.