Beat the egg yolks and 3/4 cup of the sugar in a bowl.
Heat 3 cups of the milk, along with the lemon zest and cinnamon stick, in a pot
over moderate heat just until it begins to boil. Remove the milk from the heat and
strain into the egg mixture, whisking constantly.
Dissolve the cornstarch in the remaining milk and add it to the mixture.
Pour the mixture into the pan and cook over low heat, stirring constantly, just until
it begins to bubble.
Pour into individual ovenproof serving dishes and refrigerate for at least 2
Before serving, sprinkle the remaining sugar over the top of the custards and
place under a preheated broiler until the sugar is melted, light brown and bubbling.