Dark Chocolate Semifreddo

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.

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.

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 250°F.