In a medium saucepan, combine the milk, cinnamon stick, and lemon peel. Bring the mixture to a boil over medium heat.
In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth.
Once the milk mixture has boiled, remove it from heat and discard the cinnamon stick and lemon peel.
Slowly pour the milk mixture into the egg mixture, whisking constantly to prevent the eggs from cooking.
Return the mixture to the saucepan and cook over medium heat, stirring constantly, until it thickens and coats the back of a spoon. This should take about 10-15 minutes.
Remove the mixture from heat and pour it into individual serving dishes.
Allow the natillas to cool to room temperature, then refrigerate for at least 2 hours before serving.
Before serving, you can sprinkle a little bit of cinnamon on top of each dish for added flavor.