In a saucepan large enough to hold all ingredients whisk coconut milk, cornstarch, lemon zest, lemon juice and sugar to a boil. Constantly whisk to prevent burning.
Once mixture boils and makes thick bubbles remove from heat. If preparing without egg yolk add a drop of yellow food color if desired and set pan in a bowl of ice. Stir occasionally until cold and store in a refrigerator.
If using egg yolk, whisk yolk in a small bowl, add 1/3 of boiled curd and whisk rapidly. Transfer yolk mixture back into pan and whisk well. Chill over ice and then store in a refrigerator.