Using a sharp knife, split the vanilla bean lengthwise and scrape out the seeds. In a small saucepan, combine the scraped vanilla bean and the vanilla seeds with the milk, cream and 40 grams of sugar.
Place the cornstarch or custard powder in a small bowl. Add a splash the milk mixture to it and stir to a paste. This prevents lumps. Add a splash more of the milk and stir smooth. Add the yolks and the 10 grams of sugar and whisk together. In the meantime, gently heat the milk mixture to a simmer. Once the mixture is simmering, fish out the vanilla bean and discard.
Slowly add the hot milk mixture to the egg mixture in a thin stream, whisking the egg mixture continuously. Pour the mixture back into the saucepan. Whisk over low to medium-high heat until the mixture thickens and bubbles are starting to appear. You now have pastry cream!
Pour the pastry cream through a sieve to make sure there aren’t any lumps or stray bits vanilla bean in it.
NOTE: at this point, you can serve the pastry cream as a warm dessert. If you plan to either serve the pastry cream at room temperature or cold, or use it as a filling for cakes, pies or other pastries, press a piece of plastic wrap directly onto the surface of the pastry cream to prevent a skin from forming. Let the pastry cream come to room temperature before placing it in the fridge to chill.
Pastry cream will keep in the fridge, tightly covered with plastic wrap, for up to 3 days.
Notes
I like to use custard powder when I make this pastry cream. If you can't find that, you can use an equal amount of cornstarch and about half a teaspoon of vanilla extract. Your pastry cream will be lighter in color, but it will taste just as good!
Recipe by The Tough Cookie at https://thetoughcookie.com/building-blocks-2/vanilla-pastry-cream/