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1
Combine the milk, lemon zest strips, and cinnamon stick in a saucepan over medium heat. Bring slowly to just below a simmer, then remove from the heat and allow to infuse until the milk is distinctly fragrant with lemon and cinnamon.
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2
Whisk the egg yolks, sugar, and cornstarch together in a bowl until the mixture is pale and uniform — no dry cornstarch should remain visible.
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3
Strain the infused milk through a fine sieve to remove the zest and cinnamon, then pour it in a slow, steady stream onto the yolk mixture, whisking constantly. This is where the dish lives or dies: if the milk is too hot or added too quickly, the yolks will scramble and the custard cannot be recovered — the mixture must remain smooth, pale, and fully liquid at this stage.
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4
Return the combined mixture to the saucepan over low heat, stirring continuously with a wooden spoon or heatproof spatula, until the custard thickens enough to coat the back of the spoon and holds a clear line when a finger is drawn through it.
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5
Pour the custard into four shallow earthenware dishes and allow to cool completely, then refrigerate until fully set and cold.
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6
Immediately before serving, scatter an even layer of caster sugar over each custard. Heat a hierro — a traditional flat caramelising iron — directly in a flame until it glows, then press it evenly across the sugar surface. The sugar should melt and caramelise to a deep amber sheet; the crust is done when it is uniformly golden-brown and hardens on tapping within a few seconds of cooling.