Pour the milk into a small, heavy-based saucepan and set over medium heat. Add the citrus peels and cinnamon stick, then bring to a simmer and remove from the heat.
While the milk is heating, dissolve the cornstarch in a splash of milk to make a slurry.
Beat the egg yolks and sugar with a whisk until turns pale yellow, then beat in the cornstarch slurry.
Remove the citrus peels and cinnamon stick from the hot milk, and slowly pour in the egg mixture, whisking constantly to prevent the eggs from scrambling.
Turn the heat to low and whisk constantly until the mixture has thickened to a pudding-like consistency.
Remove from the heat and divide the mixture among ramekins or traditional clay dishes. Press a piece of plastic or papel wrap onto the surface of each custard to prevent a skin from forming.
Chill the crema catalanas for about 4 hours, or preferably overnight.
Once the custards have chilled, remove the plastic wrap and sprinkle a thin layer of sugar on top of each custard. Spread the sugar out by tilting the ramekins in all directions.
Caramelize the sugar to the desired color with a small blowtorch. (For an authentic flair, use a hot iron for this step instead.) You can also place the ramekins on a baking tray on the top shelf of the oven and brown the sugar with the grill on the highest setting. Watch carefully, as they can burn in an instant!