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.
The powdered sugar is absorbed by the cake after a few hours, so additional powdered sugar may need to be sprinkled on top of the cake before serving.
Please note that some ingredients and brands may not be available in every store.