Mix sugar, margarine and cinnamon with fork until crumbly.
Place in refrigerator until ready to use.
Cream sugar and margarine together.
Add egg and beat till smooth.
Add buttermilk and vanilla.
Combine flour, soda and salt.
Add to creamed mixture and beat well.
Fold in cutup rhubarb.
Pour into a lightlygreased wellfloured 13" x 9" x 2" pan.
Sprinkle topping over cake batter.
Bake 35 to 45 minutes in 350 degree oven.