Place 5 6-ounce, oven-safe custard cups in a baking pan and set aside.
In a large bowl, whisk eggs, milk, sugar, and vanilla until well blended.
Pour mixture evenly into custard cups and sprinkle with cinnamon if desired.
Pour hot water into the roasting pan ½-inch up the sides of the cups.
Bake for 35 minutes or until a small sharp knife inserted into the center comes out clean. Custards will be slightly jiggly in the center but will set as they cool.
Remove each cup from the water bath and allow to cool completely. Refrigerate until ready to serve.