Combine butter & marshmallows in a med suacepan over low heat. Cook until marshmallows melt, about 7 mins. Stir in cocoa and Coca-cola. Remove from heat. Sift together sugar, flour and baking soda in a large bowl. Beat in chocolate-cola mixture on low speed. Add eggs & buttermilk. Beat til smooth, about 2 mins. Pour into buttered & floured cake pans and bake at 180ºC or about 40 mins.