1In large bowl,whisk together the brown sugar, egg, sour cream, oil, and buttermilk. Add the flour, baking powder, baking soda, and salt to the liquid mixture and blend everything with large spoon. (I use a wooden one) Set aside. In saucepan, saute the rhubarb with 1/4 cup water and the raspberry jello. Stir and cook until rhubarb is just tender,about 5 minutes. Take off heat and drain about 1/2 the jello water off the rhubarb. Pour in the batter and mix thoroughly.
Mix all the topping ingredients in a food processor (pulse). Pour batter in greased 9x13 baking dish. Sprinkle the topping evenly on top of batter. Bake in preheated 350 degree oven for 50-55 minutes. Check for doneness with toothpick in center.
NOTE: The raspberry jello in the rhubarb not only sweetens the rhubarb (so don't add any sugar) BUT it gives a nice pink color to the batter and finished cake.