Golden Ribbons Pasta with Zucchini and Tomato

Name: Golden Ribbons Pasta with Zucchini and Tomato
Ingredients: 3 medium yellow vine-ripe tomatoes or yellow-green tone heirloom tomatoes or 1 cup yellow grape tomatoes; 1 1/2 cups chicken stock; A healthy pinch saffron threads, about 25 threads; Salt; 1 pound egg tagliatelle; 2 tablespoons extra-virgin olive oil; 2 tablespoons good quality butter; 1 large shallot, finely chopped; 1 pound small, firm zucchini, chopped into a small dice or short thin matchsticks; 2 large cloves garlic, finely chopped; 2 tablespoons chopped fresh thyme leaves; Freshly ground black pepper; 3 large egg yolks; Freshly grated Parmigiano-Reggiano