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1
About 30 minutes ahead, cut the butter into a few pieces and set it on the counter at room temperature (65u00b0 to 75u00b0F/19u00b0 to 24u00b0C).
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2
Into a 1 cup glass measure with a spout, weigh or measure the cream. Heat in the microwave until hot, then cover it.
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3
Have ready a 2 cup glass measure with a spout, lightly coated with nonstick cooking spray, near the cooktop.
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4
In a heavy 6 cup saucepan, preferably nonstick, stir together the sugar, corn syrup, cream of tartar, if using, and water until all the sugar is moistened.
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5
Heat, stirring constantly with a silicone spatula, until the sugar dissolves and the syrup is bubbling. Stop stirring and let it boil undisturbed until it turns a deep amber and the temperature reaches 370u00b0F/188u00b0C or a few degrees below, as the temperature will continue to rise. Remove it from the heat as soon as it reaches temperature.
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6
Slowly and carefully pour the hot cream into the caramel. It will bubble up furiously.
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7
Use a silicone spatula or wooden spoon to stir the mixture gently, scraping the thicker part that settles on the bottom. Return it to a very low heat, continuing to stir gently for 1 minute, until the mixture is uniform in color and the caramel is fully dissolved.
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8
Remove the caramel from the heat and gently stir in the butter until incorporated. The mixture will be a little streaky but will become uniform once cooled and stirred.
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9
Pour the caramel into the prepared glass measure and let it cool for 3 minutes. Gently stir in the vanilla and let it to cool until room temperature and thickened, stirring it gently once or twice.
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10
Room temperature, 3 days; refrigerated, 1 month.
