2 pounds large (about 3 to 4) unpeeled russet potatoes, scrubbed
18 large egg yolks
1 cup freshly grated Parmesan (from a 3-ounce chunk)
2 cups all-purpose flour (about 10 ounces), plus extra for dusting work surfaces
1 large egg, beaten with 2 tablespoons water
Preheat the oven to 375°F. Pierce the potatoes all over with a fork and bake until a knife can easily be inserted and removed from the potatoes, about 70 minutes. Remove from the oven and let cool just enough to handle. (The goal is to quickly release the steam that has built up in the potatoes. Rubber gloves are useful when working with hot potatoes.)
Immediately peel the potatoes or scoop out their flesh with a spoon and press the flesh through a ricer. Spread the riced potatoes out onto a large baking sheet to allow the steam to escape and let cool. You should have about 5 cups total. On a work surface, mix the potatoes, cheese, and a big pinch of salt gently to combine and form the mixture into a loose mound. Make a well in the center of the mound.
Add 5 of the egg yolks to the well and break them up using a fork. Work the potato mixture into the eggs, a little at a time, until smooth. The key here is to not overwork the dough or it will become gummy. Add the flour, 1/2 cup at a time, and gently knead just until the dough becomes a large smooth ball, for about 5 minutes. (Note: Cutting the dough in half with a bench scraper and placing it on top of the other and then pressing it down will help to mix the flour in but not overwork the gluten in the flour). The dough should feel firm but yielding. To test, pinch off a piece of dough and roll it on a floured surface into a rope about 1/2-inch in diameter. If the rope holds together, the dough is perfect. If it’s too dry, add another yolk or 2 tablespoons of water to the dough. If the rope seems damp and falls apart, add more flour, knead, and test again. Cover the finished dough with a damp cloth and let it rest for 30 minutes at room temperature.
On a floured surface, divide the dough into two equal balls. Roll one dough half to 1/8 to 3/16-inch thick, flouring the dough and surface as needed so the dough doesn’t stick. With a 3 1/2-inch diameter ring mold or small bowl, cut rounds from the dough. If you need to, push together the dough scraps and re-roll until smooth. Once you have all of the rounds cut, repeat with the other half of the dough. You should end up with about 24 rounds. Sprinkle a baking sheet with flour and hold the circles on the sheet in between layers of parchment paper.
On 12 of the dough rounds, pipe a ring of the spinach filling starting 1/2 inch from the edge. Make sure to leave a 1 1/2 inch circle of dough in the center to hold the egg yolk. Carefully drop an egg yolk in the center of each round. Season the yolk with gray salt and pepper. Brush the egg-water mixture along the edges of all 24 dough rounds. Place an empty dough round over a filled one, gently sealing it all the way around, careful so as not to break the yolk inside. If you choose, crimp the edges as you would a pie. Transfer the ravioli to a floured baking sheet, cover with a damp cloth and refrigerate for at least 30 minutes. If storing for longer, up to 8 hours, cover the cloth-covered ravioli with plastic wrap.
Boil a large pot of salted water. Place one or two ravioli in the water at a time. The ravioli is ready 45 seconds after it floats to the top of the water. Take out with a spider and place on a plate.