Directions

1. 1. Preheat the oven to 350 degrees F. Prepare a nine by thirteen inch baking dish with cooking spray.
2. 2. Combine the butter, cooking oil, cocoa powder and 1/4 cup of the buttermilk in a sauce pot over medium low heat. When it comes to a boil remove it from the heat.
3. 3. In a large mixing bowl sift together the flour, baking soda, sugar and salt. Add the cocoa mixture and stir well.
4. 4. Stir together the eggs, the remaining 1/2 cup of buttermilk and the vanilla. Add to the chocolate batter and mix thoroughly; pour into the prepared dish.
5. 5. Bake for about 25 minutes or until a tester inserted in the middle comes out clean.
6. 6. To make the icing, melt the butter with the cocoa powder and milk over medium low heat. When it boils remove it from the heat. Add the powdered sugar and mix in the walnuts and vanilla.
7. 7. Remove the cake from the oven and let it cool on a wire rack for about five minutes. Cover it with marshmallows and then pour the warm icing over all.
8. 8. Let the cake cool long enough for the icing to firm up before serving.