Line a 9 × 5-inch loaf pan with plastic wrap, allowing the excess to hang over the edges. Beat in a stand mixer with the whisk on medium-high speed, 2 to 3 minutes:

Combine in a small saucepan:

Simmer over medium heat until the syrup is clear. Continue to cook until the sugar reaches 243°F on a candy thermometer. With the mixer running on medium-high speed, slowly add the hot sugar syrup to the eggs, pouring it down the side of the bowl and avoiding the whisk. Beat on high speed until light and fluffy and cooled to room temperature, 5 to 10 minutes.

Gently fold in:

Whip to soft peaks:

Gently fold the cream into chocolate mixture. Pour into the prepared pan and place plastic wrap directly on the surface. Freeze until firm, for at least 8 hours.

Remove the plastic covering and unmold the semifreddo onto a serving platter. Peel away the plastic wrap, slice, and serve immediately. Alternatively, the semifreddo may be scooped from the pan with an ice cream scoop.