In a small saucepan, combine rhubarb and sugar. Cover and cook over very low heat until rhubarb is tender.
Stir occasionally to prevent burning. Drain, discarding liquid. Let cool to room temperature.
For Cake Batter:
Preheat oven to 350 degrees. Generously butter a 9 by 5-inch loaf pan. Sift flour, salt and baking powder together.
Beat butter until creamy. Add sugar and beat until light and fluffy. Whisk eggs and vanilla together and add to butter mixture. On low speed, add flour mixture. Using a rubber spatula, fold in rhubarb. Pour batter into prepared pan. Smooth with a spatula and make a lengthwise split i inch deep to minimize splitting.
Bake until cake is browned and a tester comes out clean, about 1 hour and 10 minutes. Unmold and let cool right side up on a wire rack.
(Adapted from Williams-Sonoma Taste)