Combine flour, baking powder, soda and salt, set aside. With a mixer, blend together 1 3/4 cups of the sugar, prune puree, yogurt, oil and vanilla. Fold in mashed and diced bananas. In a clean bowl with clean beaters, beat egg whites, and cream of tartar until whites are foamy. Gradually add remaining sugar, beating until whites form soft peaks. About 4 minutes. Gently fold whites into banana mixture, then fold in flour mixture until just blended. Don't overmix or muffins will be tough. Line muffin pans with liners. Fill to top; evenly distribute walnuts over tops. Bake in 350 until muffins are just firm to the touch and edges are golden, about 25 minutes.