1. Place potatoes in large pot and cover with cold water. Add 1 teaspoon salt. Bring to a boil over high heat. Reduce heat; simmer until fork-tender, 25 minutes. Drain; return potatoes to pot; cook over medium heat 1 minute or until dry.
2. Pass potatoes through food mill over large bowl, or push through medium sieve with spatula. Add melted butter, egg, yolk, 1/4 teaspoon salt, pepper and nutmeg. Mix in enough bread flour to make stiff dough. Knead a few times in bowl to blend.
3. Heat oven to 200 degrees F. Line baking sheet with waxed paper. Lightly dust with flour.
4. Divide dough into 6 equal pieces. With well-floured hands, roll each into rope, 1 inch in diameter. Cut each rope into 1/2-inch pieces. Place on prepared baking sheet. You should have about 90 gnocchi. (See Make-Ahead Tips, below.)Cook Gnocchi:
5. Place heat-proof plate with 2 tablespoons butter in 200 degrees F oven. Bring large skillet or pot of water to a boil. Add remaining 1 teaspoon salt. Drop in about a quarter of the gnocchi. When they float to top, about 2 minutes, remove with skimmer and transfer to warm plate in oven; stir to coat gnocchi with butter. Cook remaining gnocchi in batches. Reserve 1 cup of the gnocchi cooking water.Tomato Sauce:
6. If using sauce, heat in medium-size saucepan. Add cooked gnocchi; gently heat through. Add as much of reserved pasta water as needed to thin sauce and evenly coat gnocchi.