CAKE: In a large bowl, cream together butter and brown sugar until light and fluffy. Add egg and vanilla, beat until well-blended.
In a separate bowl sift together the flour, soda and salt.
Add flour mixture and buttermilk alternately to the creamed mixture, beat after each addition, just enough to blend. Gently fold the chopped rhubarb into the batter, spoon into a greased and floured 9x13 inch pan, spreading evenly.
TOPPING: In a small bowl blend together the granulated sugar, cinnamon and nuts. Sprinkle evenly over the top of the rhubarb cake.
Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean. Cool the cake in the pan, then cut into squares and serve.
Cook's Note: Serve with a dollop of sweetened whipped cream or whipped topping.