Combine together the sugar, flour, and xanthan gum. Set aside.
In a saucepan, mix together the oil, butter, cocoa, and cola. Bring to a boil over medium heat, stirring constantly.
Add the chocolate mixture to the sugar mixture, and beat until smooth. Stir in the marshmallows, buttermilk, baking soda, eggs, and vanilla, and blend well.
Pour the batter into the greased large rectangular pan, and bake at 350°F for 40-45 minutes or until done.