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1
Combine the whole milk with the cinnamon stick, lemon zest strips, and orange zest strips in a heavy saucepan. Set over low heat and infuse for 30 minutes; the milk should steam gently and carry the scent of cinnamon and citrus throughout — it must not boil.
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2
In a bowl, whisk the egg yolks with the 150 g caster sugar until the mixture is thick and pale. Whisk in the cornstarch until fully incorporated and no dry pockets remain.
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3
Strain the infused milk through a fine sieve, discarding the cinnamon stick and zest. Pour the hot milk in a steady stream onto the yolk mixture, whisking constantly to temper without scrambling the eggs. Return the combined mixture to the saucepan.
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4
This is where the dish lives or dies: set the pan over medium heat and stir continuously with a wooden spoon or heatproof spatula, reaching every corner of the base and sides, until the custard thickens to coat the back of the spoon heavily and registers 80–85 °C on a probe thermometer. The line it cannot cross is a rolling boil — above it, the eggs curdle and the cornstarch breaks.
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5
Remove from the heat immediately. Pour into shallow, flat earthenware cassoles (the traditional terracotta vessels), smoothing the surface level with the back of a spoon. The custard should settle into a layer denser than crème anglaise but softer than pastry cream.
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6
Refrigerate uncovered until fully set — a minimum of 4 hours, overnight is preferable. The surface should feel firm and dry to the touch, neither tacky nor wet.
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7
To serve: sprinkle the surface of each cassole evenly with caster sugar to a depth of no more than 1–2 mm. Caramelize with a ferro de cremar (a circular iron disc heated over a flame until glowing and pressed onto the sugar) or a blowtorch. The crust is ready when it is mahogany-dark, glassy, and cracks cleanly when tapped with a spoon.