Preheat oven to 350 degrees F. Wrap the outside of a 9 inch springform pan with aluminum foil to prevent leaking, because it will leak. Sift together the flour, baking powder, baking soda, and salt. Set aside.

In a saucepan over medium heat, combine brown sugar and 1/2 cup butter. Bring to a boil, then add rhubarb and walnuts, stir, and pour into bottom of springform pan. Do not overcook the sugar mixture.

In a large bowl, cream together the butter and 3/4 cup sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the sour cream. Pour batter into prepared pan.

Bake in the preheated oven for 60 minutes, or until a toothpick inserted into the center of the cake comes out clean. Cool in pan for 10 minutes, then invert onto serving platter and carefully remove pan. Serve warm with whipped cream, if desired.