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.

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.

The egg custard can be refrigerated for up to 3 days.