In a medium pot add the peanut butter, water, and butter and bring to a boil. Remove from heat.
In a separate bowl, mix together the sugar, brown sugar, flour, salt, and baking soda with a fork. Mix together with the peanut butter mixture.
Add the eggs, buttermilk, and vanilla to the bowl and mix well.
Stir well and pour into greased or parchment lined cookie sheet pan with a little edge (12X17 jellyroll pan). Bake for 13-15 minutes at 375 degrees. Do not overbake!
For the Frosting:
In a small pot bring the butter, buttermilk, and peanut butter to a boil. Remove pan from the heat and add the powdered sugar and vanilla. Pour frosting over the warm cake.
Notes
Source: Nina JonesButtermilk Substitution: For every cup of buttermilk needed, add one tablespoon lemon juice or white vinegar to a liquid measuring cup, and add enough milk until it measures 1 cup. Stir, and let sit for five minutes before using.