Combine first 3 ingredients in a medium saucepan over medium heat, stirring until butter melts. Remove from heat.
Combine flour, sugar, and baking soda in a large bowl; stir in butter mixture. Add eggs, and stir until blended. Stir in buttermilk, vanilla, and marshmallows.
Pour into a lightly greased 13- x 9-inch baking pan. Bake at 350º for 25 to 30 minutes or until a pick inserted in center comes out clean. Remove from oven; spread Chocolate Coca-Cola Icing over warm cake.