A direct descendant of the Spanish Crema Catalana , natilla is a rich, creamy egg custard made without the crispy sugar topping. It's sweet, thick, comforting and perfect for entertaining, because it can be prepared ahead completely. Adding eggs to the warm milk mixture can scramble them; to prevent that, add the milk mixture to the eggs very slowly in order to increase their temperature gradually.
1. In a large saucepan, combine the milk with the lemon zest, cinnamon stick and salt and bring to a simmer. In a large heatproof bowl, using a handheld electric mixer, beat the granulated sugar with the egg yolks at medium speed until they are pale, about 4 minutes. Beat in the cornstarch slurry. At low speed, gradually beat in half of the hot milk.
2. Pour the egg-and-milk mixture into the saucepan and cook the custard over moderate heat, whisking constantly for 18 minutes, until very thick. Whisk in the vanilla. Transfer the custard to a large bowl and discard the cinnamon stick and lemon zest. Press a piece of plastic wrap directly onto the surface of the custard and refrigerate until chilled, about 3 hours. Spoon the custard into 6 bowls, sprinkle with ground cinnamon and serve.