Bring to a boil the broth, squash, walnuts, and onions. Reduce heat and cook until the squash just becomes tender. (Don't overcook or you'll wind up with mush.) Stir in the egg yolks, saffron, and salt. Stir; allow to cook for a few minutes, then remove from heat. Serve with the sugar & spice mixture as a garnish or in small serving bowls with spoons off to the side, to allow guests to season as they may.