2 cups full cream milk
1/2 cup sugar
1/2 vanilla bean, split lengthwise, seeds scraped (or vanilla bean paste or vanilla essence)
Pinch of salt
4 large egg yolks
1/4 cup cornstarch
2 Tbs unsalted butter, cut into small pieces
In a medium saucepan combine milk, 1/4 cup sugar, vanilla bean and seeds and salt. Cook until mixture comes to a simmer.
Whisk together egg yolks, cornstarch and remaining 1/4 cup sugar, continue to whisk and slowly pour 1/2 cup of the hot-milk mixture into the egg yolk. Continue to do this 1/2 cup at a time until incorporated.
Pour mixture back into the saucepan and cook over medium-high heat while whisking constantly until it thickens and is 160 C (this should take about 2 minutes - remember to remove the vanilla bean).
Pour thickened mixture into an electric mixer, add butter and beat on medium speed until the butter melts and the mixture is cool.
Cover with cling wrap, pressing it directly to the surface of the pastry cream and refrigerate until chilled (2 hours up to 2 days).
Beat it on low speed until smooth just before using - or whisk it by hand.
Credits: Martha Stewart Baking Handbook