1) Stir the milk and cornstarch together, then beat in the egg yolks. Set aside.
2) Bring the stock to a boil in a 4-qt. soup pot and add the rice. Cook, covered, until the rice is puffy and tender, about 25 minutes.
3) Remove the soup from the heat and add the milk and egg mixture, whisking continuously.
4) Place back over heat and cook for a moment until it thickens.
5) Remove from the heat again and add the butter, parsley, and lemon juice, and the optional grated lemon peel.
6) Add salt and pepper to serve.