In a saucepan, heat the milk, cinnamon stick, and lemon zest over medium heat until it just starts to simmer. Remove from heat and let it steep for 15-20 minutes.
In a mixing bowl, whisk together the egg yolks, sugar, and cornstarch until well combined.
Slowly pour the warm milk mixture into the egg yolk mixture, whisking constantly.
Return the mixture to the saucepan and cook over low heat, stirring constantly until it thickens (about 8-10 minutes).
Remove from heat and strain the mixture to remove the cinnamon stick and lemon zest.
Divide the custard into 4 ramekins and refrigerate for at least 2 hours to set.
Just before serving, sprinkle a thin layer of sugar over the top of each custard and caramelize using a kitchen torch or under the broiler.
Let it sit for a few minutes to allow the sugar to harden, then enjoy the delicious Crema Catalana!
