Preheat the oven to 350 degrees F. In a large bowl, mix the flour, baking soda, sugar, and salt; set aside. In a saucepan, bring the oil, margarine, peanut butter, and 1 cup of water almost to a boil (do not boil). Pour over the dry ingredients and mix well. Add the eggs, vanilla, and buttermilk. Mix well. Pour the batter into a greased and floured 15x11x1-inch pan. Bake for 15 to 18 minutes, or until a toothpick inserted into the center comes out clean.

Combine the evaporated milk, sugar, and margarine in a saucepan. Bring to a boil and cook for 2 minutes. Remove from the heat and add the peanut butter and marshmallows, stirring until melted. Stir in the vanilla. Pour the icing over the warm cake and spread to cover. –Martha Sparkman, Poplar Bluff, Missouri