In small bowl, sift together flour, baking soda and salt. Set aside.
In larger bowl, by hand, cream together brown sugar and butter.
Add egg and continue to beat by hand until well-blended.
Stir in vanilla.
Now, ALTERNATE between adding the 1 cup of milk and the flour mixture that you'd set aside.
Add about a 1/4 of the milk and stir well. Then add about 1/4 of the flour mixture, and stir well.
Continue until all is added to the batter.
Stir in the chopped rhubarb.
Spoon into sprayed (or greased and floured) 9 x 13 baking dish or cake pan.
Sprinkle with white sugar/cinnamon topping.
Bake 40 minutes at 350º F. (or until knife comes out clean)
Shake pan from side to side to evenly distribute sugar topping if necessary.
Serve warm with a scoop of ice cream or whipped cream.