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Put the milk in a medium saucepan and scrape in the vanilla bean seeds. Add the vanilla bean pod to the pot as well. Whisk well to break up the seeds. Just bring to a boil over medium heat, remove from the heat, and allow to steep for 15 minutes. Discard the bean pod.
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Meanwhile, whisk together the eggs, sugar, cornstarch, and salt in a heatproof bowl until very smooth.
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Drizzle about one-quarter of the warm milk over the egg mixture, whisking constantly. Add the remaining warm milk and whisk to combine. Immediately pour the mixture back into the saucepan, and cook over low-medium heat until it begins to simmer and bubbles appear around the edge. Cook, whisking constantly to prevent scorching, for about 1 minute. The pastry cream should be thick enough to see whisk marks, but still satiny smooth and fluid. Remove from the heat and whisk in the butter.
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Proceed as called for in individual recipes or cool and refrigerate until needed. See Tips below; stir the pastry cream a couple of times as it cools down to dissipate heat. When almost at room temperature, scrape into an airtight container, press plastic wrap directly onto the surface, snap on the cover, and refrigerate for at least 4 hours or until thoroughly chilled. Refrigerate for up to 3 days.