If using large tomatoes score the bottoms with an X cutting through outer skin only. Bring a small pot of water to boil over medium heat and submerge the tomatoes for 1 minute. Remove and let cool for 1 to 2 minutes. When cool enough to handle, peel, seed and chop them into a fine dice, about 1/4-inch. Reserve in small bowl. If using grape tomatoes, cut them in half and reserve.
Put the stock and saffron into a small pot over medium-low heat. Simmer gently. Bring a large pot of water to a boil over medium heat for the pasta. When the water is at a full, rolling boil, salt the water and add the pasta. Cook the pasta to al dente. Drain.
While the pasta is cooking, heat a large skillet over medium heat with extra-virgin olive oil, a couple of turns of the pan, and the butter. When the butter melts into the oil add the shallots, zucchini, garlic and thyme. Season with salt and pepper, to taste, and saute until tender, about 6 to 7 minutes. Stir in the tomatoes and let them heat through. Add the pasta and stir gently.
Beat the egg yolks in a small bowl with 1/2 cup of the saffron stock. Pour the remaining stock and tempered eggs over the pasta and turn off the heat. Toss the pasta and the sauce for 1 to 2 minutes to thicken the sauce.
Serve the pasta immediately in shallow bowls garnished with Parmesan.