Cream together the 1/2 cup of butter and the sugar.
Beat in the egg and the vanilla.
In a separate bowl, stir together the 2 cups of flour, the baking soda and the salt. Alternatively you can add these to the creamed mixture, starting and ending with the flour mixture.
Toss together the rhubarb and the teaspoon of flour and fold it into the batter.
Spread the batter into a 9 x 13 baking pan.
Mix together the butter and cinnamon and sprinkle over the batter.
Bake at 350 degrees. this is a guess, as no temperature was given with the recipe :-)