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1
Preheat the oven to 300u00b0F [150u00b0C]. Arrange eight ramekins or custard pots with a 3/4-cup [180-ml] capacity in a large, shallow baking pan. Set a fine-mesh sieve over a large liquid measuring cup or pitcher. Place the egg yolks in a large bowl and set aside.
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2
In a medium, heavy-bottomed saucepan over medium heat, cook the brown sugar and butter, without stirring, until the sugar turns a deep amber color and develops a nutty smell, 10 to 15 minutes. Pour the heavy cream into the mixture gradually (and carefully, to avoid hot spatters), whisking constantly. Remove from the heat. Add the kosher salt, and scrape the seeds from the vanilla bean into the mixture. Stir to combine.
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3
Slowly pour the hot butterscotch mixture into the egg yolks, whisking constantly. Strain the custard through the sieve into the measuring cup. Pour into the ramekins, dividing the custard evenly. Pour hot water into the baking pan until it reaches halfway up the outside of the ramekins, and cover the pan with aluminum foil. Bake for 45 minutes. Rotate the pan and continue to bake for 15 minutes more, or just until the custard has set and doesn't jiggle with you shake the pan. Remove the pan from the oven, uncover, and, using tongs, carefully lift the custards from the water bath and set on a cooling rack. (The custards can be made up to 1 week in advance and stored, covered, in the refrigerator. Bring to room temperature before serving.)
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4
To make the caramel sauce: In a small saucepan over medium-high heat, combine the granulated sugar, water, and kosher salt and cook, without stirring, until the mixture turns red-brown in color, about 5 minutes. Gradually add the heavy cream, whisking constantly until the sauce is smooth. Set aside until ready to use.
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5
In a 1-qt [1-L] jar, combine the cream and buttermilk. Partially cover and let stand in a warm spot (about 78u00b0F [28u00b0C]) until the cream tastes slightly sour and has thickened to a puddinglike consistency, 24 hours to 3 days.
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6
Store in an airtight container in the refrigerator for up to 2 weeks.
