Pour the milk in the casserole and add all the ingredients.
The cornstarch flour must be sieved to avoid the formation of lumps.
Place the casserole over medium heat and stir continuously with a spoon until it boils.
Once boiled, remove from the stove.
Remove the lemon zest, cinnamon stick and vanilla (if applicable).
Pour the hot cream into individual low ramekins. Let it cool down a little and put sugar on top to caramelize with the gas burner. It's ready to serve!