In the bowl of a stand mixer fitted with a paddle, beat the egg yolks on low speed. Slowly add the sugar, followed by the salt and vanilla bean seeds, and increase the speed to medium-high. Beat the egg-sugar mixture until very thick and pale yellow, about 5 minutes. Scrape down the sides of the bowl and add the cornstarch. Turn the mixer to low speed and mix until combined.

In a small saucepan over medium-low heat, heat the milk, heavy cream, and vanilla bean pod until just about to simmer. Remove the pan from the heat and pour the mixture into a medium measuring cup with a pourable spout.

With the mixer running on low speed, very slowly add the hot milk mixture (along with the pod). Mix until completely combined.

Transfer the mixture to a medium, heavy-bottomed saucepan. Cook over low heat, stirring constantly with a wooden spoon, until the pastry cream becomes very thick and begins to boil, 5 to 7 minutes. Switch to a whisk, and whisk the mixture until the pastry cream thickens and is glossy and smooth, 3 to 4 minutes. Remove the pan from the heat and strain the pastry cream through a fine-mesh sieve into a medium bowl. Discard the vanilla bean pod. Stir in the butter and vanilla. Cover with plastic wrap, making sure the wrap sits directly on top of the cream (this will help keep it from forming a skin). Place in the refrigerator until well chilled. Pastry cream can be kept refrigerated in an airtight container for 4 to 5 days.