Heat oven to 350F. 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 milk, lemon juice, eggs and soda; blend. Add vanilla and marshmallows. Pour into greased and floured 9x13-inch baking dish.
Bake for 35 to 45 minutes. Cool cake in pan. Dust with powdered sugar.