In a medium bowls, combine the brown sugar, egg, vanilla, milk, and shortening.
In separate bowl, sift together the flour, salt, and soda. Then add rhubarb. Combine both mixtures together.
Pour into a prepared 9x13 pan. Sprinkle with the cinnamon sugar mixture
Bake 35 - 40 minutes
Notes
*I've seen sour milk made a couple different ways. Either by using 1 full cup of milk then adding the vinegar or, adding the vinegar first and then adding enough milk to make the total liquid equal one cup.