Cream sugar and margarine. Add egg and salt. Stir in milk, soda, vanilla and flour. Add rhubarb.
Pour in 9 x 13 pan
Sprinkle with 1/4 cup sugar and 1 tsp. cinnamon
Bake 375 deg for 35 minutes.
Re. buttermilk. I find recipes always call for so little and then it gets wasted so now I freeze the balance in small containers. It works very well. This recipe is not the same without the sour/buttermilk. I also use buttermilk to soak chicken overnight if I am going to fry it. That make a difference too, it must be a chemical reaction or something.
Now all you need it next years rhubarb harvest lol