1*If you don't have buttermilk, you can use this substitute.
In a 1 cup measuring cup, add 1 tablespoon of lemon juice (fresh or bottled). Fill the cup up to the 1 cup line with regular milk. Wait 5 minutes and use in the recipe in place of buttermilk.
2Preheat oven to 375. Prepare pan(s) by greasing either 2 - 8 inch pans, or 1 regular rectangular cake pan.
3In a large bowl, cream together butter and sugar. Add in egg and salt and mix well. Beat in milk, baking soda, vanilla, cinnamon, and flour until well mixed. Mix in rhubarb on low until well incorporated.
4Pour into prepared pans. Mix together your sugar/cinnamon topping in a separate bowl.
5Sprinkle topping evenly over top of batter in pan(s). I usually don't end up using all of the topping. Use your own discretion.
6Bake at 375 for 20-25 minutes for round pans, or 24-28 minutes for rectangular pan, until a toothpick inserted in the center comes out clean. Let cool.
7Serve topped with Cool Whip (or whipping cream), or vanilla ice cream. (Optional)