Mix the sugar, egg yolks, custard and sauce powder until frothy. In the meantime, bring the water to the boil together with the lemon juice and the grated zest, then slowly add the foamy mixture and bring to the boil.
Prepare egg whites from the egg whites, let the egg yolk mixture cool down and fold in the egg whites. Put in a glass bowl or dessert bowl (in portions), decorate if necessary. Chill in the refrigerator for a while and enjoy.