Use a small paring knife to slit open the vanilla bean half.
Open the bean and press flat with your knife, scraping the vanilla bean seeds onto your knife edge.
Add to 3 tablespoons sugar, and use your fingers to break up the seeds and combine with the sugar.
Put the vanilla sugar, empty vanilla bean pod, and 1 cup milk into a saucepan.
Heat over medium, whisking to dissolve the sugar.
Meanwhile, put egg yolks in a stand mixer with the whisk attachment and set to medium speed.
Slowly add 3 tablespoons sugar, then whip together on high speed. Scrape down the mixer bowl as needed.
When egg mixture gets to ribbon stage and lightens (about 3-4 minutes), stop whipping.
Meanwhile, whisk cornstarch into the remaining ⅓ cup of milk until smooth. Set aside.
When milk in the saucepan has just come to a low boil (watch carefully), add the cold milk mixture to the stand mixer and combine on low.
Once it is well combined, slowly pour in the hot milk with mixer on low. As soon as it is incorporated, turn off mixer and pour the entire mixture back into your saucepan.
Over low heat, stir with a whisk constantly until mixture comes to a boil again and starts to thicken (about 2 minutes).
Then whisk for another 2 minutes over low heat to make sure any raw taste from the cornstarch is gone. Whisk aggressively if it looks like the mixture isn’t coming together smoothly.
Whisk in butter, then turn off the heat. Remove the vanilla bean.
Scrape pastry cream into a bowl and press plastic wrap directly over the top. When cool, put in the refrigerator to store.
Notes
Storage Tips: Pastry cream will keep for up to 3 days in the refrigerator if you are going to use it without baking it. If you are using it in a baked recipe, it will keep for 4 days in the refrigerator.Expert Tips: This recipe fills a 9-inch tart shell. You can easily double or triple this recipe if you need to make a larger amount. Timing in this recipe is very important. Don’t be tempted to whip the egg yolks and sugar together in advance, because the sugar can begin the curdle the yolks if it sits and is not used right away. When you add the hot milk to your mixer bowl, you are tempering your eggs. This should be done slowly, while keeping the egg mixture moving constantly while you are adding the milk. That will ensure you don’t cook the eggs and end up with cooked egg bits in your custard. If the pastry cream begins to boil and your heat is too high, or you aren’t stirring fast enough, your pastry cream may lump. Stir vigorously, or pass the custard through a fine-mesh sieve. You can also use this technique if your crème pat gets lumpy after it has cooled (or after it has been stored in the refrigerator). Put it into a bowl and whisk it vigorously or pass through the fine-mesh strainer.