In a mixer add butter and brown sugar; beat. Add egg and vanilla; beat. Add buttermilk; beat. Add baking soda, salt and spices; beat. Add flour; beat until smooth. Add rhubarb; stir. Pour batter in greased 9x13 pan.
2In a separate bowl add sugar, coconut, pecans and cinnamon; stir. Sprinkle the coconut topping evenly over the top of the cake.
3Bake 35-45 minutes; test with a toothpick for doneness.