Mix the applesauce, brown sugar, and egg, then mix in the milk & vanilla.
Add the flour, salt, and baking soda and mix well combined and fold in the chopped rhubarb.
Pour in to a greased 9 inch cake tin.
Mix together the extra sugar and cinnamon, and scatter evenly over the cake batter.
Bake for 60-70 minutes, or until a skewer comes out clean.
Notes
Sadly I never met my Mother in Law as she passed before I married Lord D. I did take care to bring her rhubarb plant home with me which blesses me each year with a great yield of fresh all summer long. I give some to friends and neighbors and have a number of rhubarb recipes from the Edwardians which I have adapted.