INGREDIENTS
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1 cup sugar
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2 tablespoons cornstarch
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2 tablespoons unsweetened cocoa powder, preferably Dutch-process
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 1/4 teaspoon kosher salt
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4 large egg yolks
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2 cups whole milk
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1 cup heavy cream
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 1/2 vanilla bean, split lengthwise
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6 ounces bittersweet chocolate, melted, slightly cooled
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1 tablespoon unsalted butter
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1 teaspoon vanilla extract
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Whipped cream and chopped pretzels (for serving)