Pour 850 ml of the milk into a heavy-bottomed saucepan. Add the cinnamon stick, lemon zest, and orange zest. Heat gently until the milk is steaming and just beginning to simmer. Remove from the heat, cover, and leave to infuse for 10 minutes.

In a large bowl, whisk the egg yolks with 160 g of the sugar until smooth and slightly lighter in color.

Mix the cornstarch with the remaining 150 ml of cold milk until completely smooth. Add this mixture to the egg yolks and whisk until combined.

Remove the cinnamon stick and citrus peels from the warm milk. Gradually pour the infused milk into the egg mixture, whisking continuously so the eggs do not scramble.

Pour the mixture back into the saucepan through a fine-mesh sieve.

Place the saucepan over medium-low heat and stir continuously with a whisk or silicone spatula, scraping the bottom and corners of the pan. Heat until the custard begins to bubble gently and thickens noticeably, then cook for another 30–60 seconds while stirring constantly. Do not allow it to boil vigorously.

Remove the saucepan from the heat. If using Cointreau, stir in 1 teaspoon now, mixing until evenly incorporated.

Divide the custard between eight shallow ramekins or traditional Catalan earthenware dishes.

Allow the custards to cool briefly, then refrigerate promptly for at least 3 hours, until completely chilled and set. Cover once they have stopped steaming.

Just before serving, sprinkle the remaining 40 g of sugar evenly over the eight custards, using approximately 1 teaspoon per portion.

Use a kitchen blowtorch to caramelize the sugar until it melts and turns a golden amber color. Keep the flame moving to avoid burning the sugar.

Allow the caramelized topping to harden for 1–2 minutes before serving. Enjoy immediately while the sugar crust is crisp and the custard underneath is cold.