Preheat oven to 400 degrees. For the cake, in a medium bowl combine the sugar and flour. In a large mixing bowl mix eggs, baking soda, and sour cream well. Set aside. In a small saucepan combine peanut butter, butter, and water. Bring to a boil. Stir the boiled peanut butter mixture into the flour mixture. Add to the sour cream mixture. Mix well. Pour into a lightly greased 11X17 inch sheet/jellyroll pan.
Bake 20 minutes or until a toothpick inserted in the center comes out clean.
For the frosting, while the cake is baking, combine the butter, peanut butter, and milk in a saucepan. Bring to a boil stirring constantly. Add the powdered sugar and mix well. Stir in the vanilla. The frosting will be thick. Mix well. Pour and spread over cake while both are still warm.
Let the cake and frosting cool completely. Cut into pieces and serve.