Boil eggs for exactly 10 minutes. Put eggs in cool water and boil both together, otherwise a green ring will form around the yolk.
When done perfectly, the yolks should still be quite soft even when the whites, cooked. The Cantonese call them tong sum dan, which literally translates to sugar heart eggs.

Egg whites cook at 55 °C and the yolks, at 70°C, so it is not a good idea to cook eggs in boiling water. Instead, lower them into a pot of simmering water and dunk them into cold water after exactly 4 minutes to stop the cooking process. It will otherwise continue to cook and a green ring will form where the y0lk meets the white.