Preheat the oven to 350 degrees F if using a metal pan, or 325 F if using glass.

Grease and flour a 13x9 inch pan. If using a glass pan, only grease the bottom.

In a small bowl, mix together the flour, cocoa powder, baking soda and salt.

In a mixing bowl, beat the butter until creamy. Add the sugar and vanilla and continue beating until creamy, then beat in the eggs, one by one.

Combine the cola and the buttermilk and add to the batter alternately with the flour mixture, stirring until smooth. Stir in the marshmallows. Pour the batter into the pan and bake for about 30-35 minutes at 350 if using a metal pan, and 45 minutes at 325 if using a glass pan. Use a toothpick test to see if it's done. It still looks a bit shiny when done, so you need to poke something into it and check for moist crumbs (as opposed to batter).

Let cake cool for 10 minutes, then start making the icing.

Put the butter, cocoa and cola in a large (I use a 3 quart) heavy saucepan and bring to a boil over medium heat. Remove from heat and stir in sifted sugar and vanilla until smooth.

Pour hot frosting over cake. Allow the frosting to set and then serve from the pan with vanilla ice cream or let the cake cool completely and serve it the next day.