In saucepan, mix butter, coke, oil and peanut butter. Bring to boil, but do not cook. Remove from heat and add marshmallows. Stir until dissolved, then let cool. Mix together flour, sugar, soda, eggs, buttermilk, and vanilla, then add peanut butter mixture and mix again. Pour into a greased and floured 9x13 pan. Bake at 325 for 45 minutes. Put icing on cake immediately when it comes out of the oven and is still hot.