For the cake:
Preheat oven to 350º. Grease and flour a 9×13–inch baking pan. Set aside.
In a medium-size bowl, whisk together flour, baking soda, and salt, set aside.
In a large mixing bowl, cream together butter and brown sugar until fluffy, 4-6 minutes. Beat egg in.
Add a third of the flour mixture and stir gently. Add half the sour cream and stir. Repeat with flour mixture, remaining sour cream, and remaining flour mixture. Stir in vanilla and rhubarb and spread batter into prepared pan.
In a small bowl, combine sugar, nuts, cinnamon, and melted butter; sprinkle over the top of the cake. Bake 40 to 50 minutes.