Heat oven to 350° F. Combine flour and granulated sugar in large bowl. In small saucepan combine butter or margarine, cocoa, and cola; bring to boil. Pour over flour mixture and stir. Add buttermilk, eggs, and soda; blend. Add vanilla and marshmallows. Pour into greased and floured 9 x 13-inch baking dish. Bake for 35 to 45 minutes. Cool cake in pan. Dust with confectioners' sugar.