Put the milk on the stove in a small saucepan, along with the citrus peels and the cinnamon stick. Slowly bring to a boil.
Dissolve the cornstarch in a splash of water and set aside.
While the milk is slowly heating, beat the egg yolks with the sugar, until the mixture turns pale yellow. Beat in the dissolved cornstarch and a spoonful of the hot milk.
Remove the citrus peels and cinnamon stick from the hot milk, and lower the heat.
Slowly add the egg yolk mixture to the milk, making sure to keep stirring so that the eggs don't scramble!
Stir constantly over a low heat, until the mixture has thickened.
Remove from the heat and pour the mixture into traditional clay dishes or ramekins.
Allow the custard to cool, and then cover in plastic wrap and chill the crema catalanas for about four hours (preferably overnight).
Before serving, I like to let mine come to room temperature (unless it's summer-- that would be too hot!). Then sprinkle a thin layer of sugar on top of each ramekin. Spread the sugar out by tilting the ramekins in all directions. Caramelize with a small kitchen blowtorch (though the truly authentic crema catalanas are made with a hot iron (see photo below).
Top with fresh fruit (optional) and enjoy!