Topping: Blend butter, cinnamon and brown sugar together and set aside.

Batter: Cream margarine and sugar together. Beat in egg and vanilla.

stir 2 cups flour, baking soda and salt together; add to creamed mixture alternatively with buttermilk, beginning and ending with flour.

Toss rhubarb with one tbsp flour and fold into batter. Spread batter in a greased bakin g pan.

Sprinkle topping over batter. Bake at 350°F for 55 minutes or until cake tests done.

Let cool in pan on wire rack