1. Preheat oven to 350 degrees F.
2. Mix buttermilk, melted butter, egg, and vanilla. Stir in rhubarb.
3. In separate bowl, mix dry ingredients, then mix with rhubarb mixture.
4. Spread batter into a greased 13 x 9 inch pan. Sprinkle with brown sugar and nuts. Bake about 45 minutes.
5. Heat topping ingredients in a small saucepan over low heat until butter is melted.
6. Poke holes in the warm cake and drizzle the topping over cake.