I made a simple pastry cream from a recipe that I've used once before uneventfully, and this time it turned out with a grainy texture.
The procedure was basically to: whisk together 3 egg yolks and an egg together with sugar and cornstarch in one bowl, separately bring milk with butter to a boil, temper the egg mixture with the hot milk, and then add the warm egg mixture to the milk and boiling for three minutes.
What might I have done wrong that would cause a grainy temperature? Did I not get the eggs warm enough before adding to the boiling milk? Did I boil for too long or too short a time? Are there any other potential culprits I'm not considering.