Preparation

• Remove the tough stems from the spinach and clean by rinsing in cold water. In a 12-inch sauté pan over medium heat, melt 2 Tbs. of butter; add the spinach and cover to wilt, 3 to 5 minutes. Remove the cover and let the spinach dry out a little, then season with 1/2 tsp. salt and 1/4 tsp. pepper.In the bowl of a food processor, combine the ricotta, Parmigiano Reggiano, nutmeg, lemon zest, spinach, and a pinch of salt and pepper, and puree until smooth. Fill a pastry bag fitted with a round tip with the ricotta mixture.

  Divide the dough into 4 pieces. Using a pasta machine, roll the dough out, starting at the widest setting and ending with the second-thinnest setting. Lay out one sheet of pasta and pipe the ricotta mixture into circles on the dough, 1 inch wide and 1/2 inch tall. Place one egg yolk in the center of each ricotta circle. With a pastry brush, brush egg white onto the dough around the filling, being careful not to break the yolk. Press the dough together and make sure to remove all air from inside. Cut out the ravioli with a round fluted cookie cutter.

  Bring 6 quarts of salted water to a soft boil. Working 3 or 4 at a time, gently place the ravioli in the boiling water, cook for 3 minutes, and then remove with a slotted spoon. Repeat for the remaining ravioli. Reserve 1/4 cup of the pasta water.

  In a 2- or 3-quart saucepan, melt the remaining 8 Tbs. butter until it begins to foam. Whisk in the reserved pasta water and cook for 1 minute to emulsify the liquids.

  Place one raviolo on each individual plate, drizzle with the butter sauce, and sprinkle with grated Parmigiano Reggiano. Serve immediately.