Steam salted egg yolk until cooked. Slice into bite-size pieces. Cut century eggs into cubes.
Lightly whisk eggs with a fork and add in seasoning (A); mix thoroughly. Add century egg and salted egg yolk. Pour mixture into a heat-proof dish and steam egg mixture over rapid boiling water for 12 to 15 minutes or until egg is just set.
Put 1 to 2 tbsp oil in a wok, fry prawns and add in seasoning (B) and the rest of the ingredients. Fry well till fragrant. Dish out and pour over steamed egg custard before serving.