Whisk the cream, milk, sugar and salt in and medium sauce pan and bring to a simmer, over medium heat.
Beat the egg yolks in a medium bowl using a whisk.
Slowly whisk the hot cream into the beaten eggs yolks a half cup at time.Pour the tempered mixture back into the sauce pan. Then place the pan back on medium heat. Cook stirring constantly with a wooden spoon, until the mixture thickens. You will know when its getting thick if it coats the back of the spoon or if the mixture reached 180 degrees fahrenheit. This step should take no more than 10 minutes.
Remove the mixture from the heat and place in an ice bath. Placing the mixture in an ice bath will speed up the cooling process. While the mixture is cooling, stir occasionally with a wooden spoon until completely cooled.
Once the mixture is completely cooled, add the vanilla extract and grapenut cereal then stir to combine.
Churn/Freeze the ice cream according to manufacturer's instruction.