Whisk together the cornstarch, salt, sugar, and cinnamon in a large saucepan. Stir in the milk to evenly blend ingredients, and set over medium-high heat. Continue whisking and cooking until custard reaches a thick consistency, 25 to 30 minutes. It is important to stir the entire time, or the custard can easily burn or clump. The custard will continue to thicken as it cools. Spoon into bowls to serve.