Melt ½ cup sugar and caramelize a round pan (4 cups, capacity). Set aside.
Place together in a small pan rest of sugar, water, and lemon peel and bring to a boil.
Continue boiling until syrup reaches the soft ball stage (238° F - 113° C on a candy thermometer).
Beat yolks and egg until thick and light lemon colored. Gradually add syrup while beating.
Pour the mixture in caramelized pan.
Bake in oven over hot water at 350° F (180° C) for 30 minutes or until cake tester comes out clean when inserted. Remove from oven and cool. Unmold.
Optional: Serve with chantilly cream.