Preparation

• Thoroughly chill a large metal bowl and the beaters of an electric hand mixer. Line the bottom and long sides of a 9×5-inch loaf pan with a 12×20-inch piece of plastic wrap, leaving 4-inch overhangs on the long sides. Smooth the plastic along the sides and into the corners; it’s OK if there are wrinkles. The plastic will not completely cover the short sides.

Whip the cream

• Beat the cream in the chilled bowl with the chilled beaters on medium-high speed just until firm peaks form, 1 to 2 minutes. Refrigerate.

Make the zabaglione

• In a 4-quart saucepan, bring 1-1/2 inches of water to a boil over high heat and then lower the heat to maintain a gentle simmer. Put the chocolate in a medium metal bowl and set the bowl over the pan of simmering water; make sure the bottom of the bowl doesn’t touch the water. Melt the chocolate, stirring occasionally, until smooth. Remove the bowl from the pan and let the chocolate cool slightly, stirring occasionally.
• Clean and dry the beaters. Put the egg yolks, 1/4 cup of the sugar, and the milk in a large metal bowl. Set the bowl over the pan of barely simmering water and beat on medium speed, frequently scraping down the sides of the bowl with a silicone spatula, until the zabaglione is thick, almost doubled in volume, and the beaters leave a trail when you lift them, about 5 minutes. Remove the bowl from the pan and fold the melted chocolate into the zabaglione until combined. It’s OK if it looks grainy at this point; it will smooth out later. Set aside.

Make the meringue

• Clean and dry the beaters. Return the pan to the heat and maintain the water at a gentle simmer. Put the egg whites, the remaining 1/4 cup of sugar, the cream of tartar, and salt in a large metal bowl and set it over the pan of water. Beat on medium speed, occasionally scraping down the sides of the bowl with a clean silicone spatula, until light, fluffy, and shiny, about 3 minutes. Remove the bowl from the pan and continue beating until the meringue is very thick and billowy, about 2 minutes more.
• In a small bowl, use a fork to mash the cherry jam until loosened a bit; set aside.

Fold and freeze

• Use a silicone spatula to gently fold the zabaglione into the meringue, then fold in the whipped cream until no streaks remain. Dollop the jam over the top and fold it in with just a few long, gentle strokes to create swirls.
• Scrape the mixture into the prepared pan and smooth the top with an offset spatula, scraping off excess if necessary to create a level top. Wrap the overhanging plastic over the top to cover and freeze for at least 6 hours and up to 3 days.

Unmold and serve

• Peel back the plastic wrap from the top of the semifreddo. Invert the pan over a cutting board or serving platter. Lift off the pan, holding the overhanging plastic down on one side and then the other. Remove the plastic wrap. If the semifreddo looks wrinkled, warm a long knife or small offset spatula under hot running water, wipe the blade dry, and run it over the wrinkles to smooth them out.
• Slice the semifreddo crosswise into 1-inch-thick pieces to serve.