Preheat oven to 350 degrees. Grease and flour a 9-by-13-inch pan.
Combine sugar, flour, baking soda and optional pudding mix in bowl.
In a saucepan over medium heat, bring butter, vanilla, cocoa, water, buttermilk and eggs to a rolling boil, stirring occasionally.
Pour over dry ingredients and mix well.
Bake 25 minutes, until center bounces back when lightly touched.
Cool cake completely. Combine peanut butter and peanut oil in bowl and spread over cake. Sprinkle with marshmallows.
Prepare Chocolate Fudge Frosting:In a saucepan over medium heat combine butter cocoa buttermilk vanilla and *optional Karo. Bring to a boil, stirring constantly. Remove from heat and stir in confectioners' sugar. Stir or whisk until smooth. While still warm, pour over evenly over marshmallows.
Allow frosting to set 5-10 minutes before serving.