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Place a fine mesh sieve in a heat-safe bowl and set aside.
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Pour milk into a small saucepan. Split the vanilla bean lengthwise and use the tip of a sharp knife to scrape the seeds into the milk. Add the salt.
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Turn heat to medium-high and bring milk mixture to just under a boil, stirring occasionally.
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Meanwhile, mix the cornstarch and sugar in a medium bowl. Add the egg and whisk until smooth.
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When the milk is ready, ladle ⅓ of the milk into the egg mixture, whisking constantly.
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Pour the egg mixture into the saucepan with the hot milk and continue whisking over medium-high heat until thick. There should be a few slow bubbles in the mixture, but don’t bring it to a full boil.
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Remove from the heat and pour through the sieve.
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Cool for 2-3 minutes, stirring occasionally, until the cream is about 140 F.
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Cut the butter into 1 tablespoon pieces and mix into the cream, one at a time, stirring until smooth.
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Press a piece of plastic wrap directly on top of the cream to help it cool quickly and place in the refrigerator.