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1
Combine chocolate, vanilla, and 1/4 tsp. salt in a medium heatproof bowl. Heat 1 1/2 cups cream in a small saucepan over medium until barely simmering. Pour hot cream over chocolate mixture and let sit until chocolate is melted, about 5 minutes. Whisk chocolate mixture until combined and smooth. Chill, stirring occasionally, until cold, about 1 hour.
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2
Beat egg whites and a pinch of salt on high speed in the bowl of a stand mixer fitted with whisk attachment until soft peaks form.
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3
Meanwhile, cook sugar and 1/3 cup water in a small saucepan over medium-low heat, stirring, until sugar is dissolved. Clip thermometer to the side of saucepan; increase heat to medium. Bring to a boil without stirring and cook until thermometer registers 250u00b0F.
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4
Working quickly and with motor running, pour syrup into egg whites in a steady stream, avoiding whisk so syrup doesn't splatter. Beat until glossy, stiff peaks form (be careful not to overbeat).
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5
Using an electric mixer, beat chilled chocolate mixture until soft peaks form. Gently fold in meringue, leaving a few streaks. Scrape into a large loaf pan and cover. Freeze until solid, at least 3 hours and up to 3 days.
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6
Just before serving, let semifreddo sit at room temperature 15 minutes. Beat remaining 1 cup cream in a small bowl until soft peaks form. Serve scoops of semifreddo in small bowls with whipped cream.
