Mix sugar, flour and soda. Set aside. In a heavy saucepan, mix butter, cocoa, water, buttermilk, eggs and vanilla and bring to boil. Pour mixture over dry ingredients and mix well. Pour into generously-greased and floured 9x13 pan. Bake at 350 for 25 minutes.
I found that when we cooked this cake, it needed about 45 minutes to completely bake.
For the filling, mix peanut butter and oil together and spread over cooled cake.
For frosting, combine butter, cocoa and buttermilk in a small saucepan and bring to a boil. Pour mixture over sugar and mix well. Add vanilla and stir well. Frost cake.