Preheat the oven to 350 degrees F. Grease and flour a 13 by 9-inch baking pan.
Combine the sugar, salt, and flour in a large mixing bowl. Bring the butter, oil, cocoa, and 1/4 cup water to a boil in a saucepan. Add to the flour mixture.
Beat together the eggs, baking soda, buttermilk, and vanilla. Add to the chocolate mixture, mix well, and pour into the prepared pan. Bake for 25 minutes.
While the cake is baking, make the icing by melting the butter in the cocoa and milk over low heat. Bring the mixture to a boil, then remove from the heat. Stir in the confectioners' sugar. Slowly mix in the nuts and the vanilla. Take the cake from the oven, and when it cools a bit cover it with miniature marshmallows. Pour the warm icing over the cake and the marshmallows. Cool the cake before serving.