Step 1: Infuse the Milk

1. Pour the milk into a medium saucepan.
2. Add the lemon zest, orange zest, and cinnamon stick.
3. Heat the milk over medium-low heat until it reaches a simmer. Remove from heat and allow it to steep for 10–15 minutes, letting the flavors meld.

Step 2: Prepare the Custard Base

1. In a separate bowl, whisk the egg yolks and sugar until the mixture becomes pale and creamy.
2. Gradually sift in the cornstarch, mixing until smooth and lump-free.

Step 3: Combine and Cook

1. Strain the infused milk to remove the zest and cinnamon stick.
2. Slowly pour the warm milk into the egg mixture, whisking constantly to prevent curdling.
3. Transfer the custard mixture back to the saucepan and cook over low heat, stirring continuously. The mixture will gradually thicken; avoid letting it boil.

Step 4: Chill the Custard

1. Once the custard reaches a pudding-like consistency, remove it from the heat and pour it into individual ramekins.
2. Let the custards cool to room temperature before refrigerating for at least 4 hours.

Step 5: Caramelize the Sugar

1. Just before serving, sprinkle a thin, even layer of sugar over each custard.
2. Use a kitchen torch to caramelize the sugar until golden and crisp. Alternatively, place the ramekins under a broiler.