In a large stockpot, cover the potatoes with water and bring to a boil. Cook until they are easily pierced with the end of a paring knife, about 15 minutes. Drain and, when cool enough to handle but still hot, peel them and run them through a food mill or ricer. Set aside.
Combine the onion and pancetta in the bowl of a food processor and pulse until both are finely chopped and mingled. In a 12 to 14-inch saute pan, heat the olive oil over high heat and add the pancetta and onion mixture. Cook for 5 minutes over high heat, then stir in the potatoes and season with salt and pepper, to taste. Remove from the heat and turn the mixture into a large bowl. Mix in the egg yolks, chives, marjoram and 1 tablespoon grated Montasio. Set aside to cool.
Unwrap the pasta dough and, using a pasta rolling machine, roll it out to the thinnest setting. Cut 3-inch squares from the dough. Using a spoon or clean hands, place walnut-sized dollops of the potato mixture onto half of the squares, and top them with the other squares to form the ravioli. Seal the edges of each raviolo and place them on a baking sheet.
Bring 6 quarts of water to a boil and add 2 tablespoons salt. Cook the ravioli in the boiling water until cooked through and tender, about 4 to 5 minutes. You may need to work in batches.
Meanwhile, in a 12 to 14-inch saute pan, heat the butter over medium-high heat until it foams, subsides, and begins to brown. Add a splash of the pasta cooking water. Add the cooked ravioli to the pan and toss gently over high heat for 1 minute. Sprinkle with the poppy seeds and remaining cheese and serve immediately.
As you expand the well, keep pushing the flour up from the base of the mound to retain the well shape. The dough will come together when half of the flour is incorporated.
Start kneading the dough with both hands, using the palms of your hands. Once you have a cohesive mass, remove the dough from the board and scrape up and discard any leftover bits. Lightly reflour the board and continue kneading for 6 more minutes. The dough should be elastic and a little sticky. Wrap the dough in plastic and allow to rest for 30 minutes at room temperature.
Roll the pasta out on a pasta rolling machine to the desired thickness.