Preheat the oven to 350°.
In a large pan over medium heat, scald the milk.
Stir in the grapenuts cereal, then remove from the heat and let it cool slightly. Set aside.
In a medium bowl, beat by hand the eggs, salt, vanilla extract, and sugar and pour it into the milk mixture.
Stir to blend everything together then pour this into a two-quart casserole dish.
Sprinkle the cinnamon, or nutmeg over the top.
Place the casserole dish into a water bath. (Put the casserole dish into another larger pan filled with hot water one-quarter of the way up.)
Bake at 350° for 60 minutes.
Remove the pudding casserole dish from the water bath and let cool on a wire rack. When it is cooled, cover it and put it into the refrigerator for a few hours to chill before serving.
Serve the cold, with or without whipped cream.
Store it covered in the refrigerator.