Bring the broth to a boil, sprinkle in the rice, and cook over low heat for 15-18 minutes until tender.
Add the lemon zest, and cook for another 2-3 minutes.
When serving, mix the egg yolks, the butter and the sour cream smoothly, drizzle in some of the hot soup to warm it, then stir it into the soup - don't boil it any further.
Season with lemon juice to taste.
Serve with toasted, dry bread cubes.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.