Preheat oven to 350 degrees.
Sift together flour, sugar, salt and baking powder in a large bowl, then set aside.
Combine butter, water and peanut butter in a saucepan over medium heat and bring to a boil. Add in the flour mixture and whisk together, then set aside.
Combine egg, vanilla and buttermilk and whisk together, then transfer to peanut butter mixture and mix well until everything is just incorporated.
Spread into a greased 9x13 pan. This cake will rise so when you put in it pan, it will appear as a thin layer.
Bake for 12-17 minutes, until it springs back when gently touched.