Natillas de Canela: Spanish Cinnamon Custard

Pour the milk into a medium saucepan. Add the cinnamon stick and the strip of lemon zest, if using. Set over medium-low heat and warm gently until small bubbles begin to tremble around the edge of the pan, about 4 to 5 minutes. Do not let it boil. Remove from the heat and let the aromatics steep for 5 minutes.

While the milk infuses, whisk the egg yolks, sugar, and cornstarch in a heatproof bowl until smooth and pale, about 1 minute. The mixture should be completely lump-free; this is what keeps the finished custard silky rather than pasty.

Fish the cinnamon stick and lemon zest out of the warm milk. Pour about 120 ml (1/2 cup) of the warm milk into the yolk mixture in a slow, steady stream, whisking constantly. This raises the temperature of the eggs gently so they thicken the custard instead of scrambling it.

Pour the tempered egg mixture back into the saucepan with the remaining milk. Set over medium-low heat and cook, whisking constantly and reaching into the corners of the pan, until the custard thickens enough to coat the back of a spoon, about 6 to 8 minutes. It should be glossy and smooth, never bubbling hard.

Remove the pan from the heat and stir in the vanilla extract, if using. For the smoothest texture, pour the custard through a fine-mesh strainer into a large measuring jug or a clean bowl, pressing gently with a spatula to push it through.

Divide the custard among 4 small bowls or ramekins. Press a piece of cling film directly onto the surface of each custard to prevent a skin from forming, then refrigerate for at least 2 hours, or until cold and set. Dust with ground cinnamon just before serving and top each portion with a biscuit, if you like.