In a medium saucepan, heat 3 cups milk with zest and cinnamon stick over medium heat (about 180°F / 82°C) until steaming but not boiling. Remove from heat, cover, and let infuse for 5 minutes.
In a mixing bowl, whisk 4 egg yolks, ½ cup sugar, and 3 tbsp cornstarch until pale and smooth.
Slowly whisk the warm milk into the yolk mixture. Return everything to the saucepan and cook over medium‑low heat (170–175°F / 77–80°C), stirring constantly, until thick enough to coat the back of a spoon (5–8 minutes).
Pour the custard into ramekins, cover with plastic wrap, and refrigerate for at least 2 hours until set.
Sprinkle 1–2 tsp sugar over each chilled custard. Using a kitchen torch (or broiler), caramelize until golden and crisp. Serve immediately.
