Line 1 quart (9x4 inch) loaf pan with plastic wrap and place it in your freezer to chill while you make the egg-and-cream mixture.
In a bowl, beat the cream with an electric mixer on high until doubled in volume and holds soft peaks when the beaters are turned off and lifted up.
Set aside at room temperature, away from the stove’s heat.
Bring about 3 inches water to a boil in a large saucepan set over high heat; decrease heat so the water simmers gently.
Clean and dry the beaters for your mixer; place the egg yolks and ¼ cup of sugar in a bowl that will fit snugly over the pan with the simmering water.
Beat away from the heat at medium speed until thick and pale yellow; about 1 minute.
Set the bowl over the saucepan and continue beating until the mixture will make little mounds when the beaters are turned off, lifted up, and any mixture on them is allowed to dribble back into the pan, about 2 more minutes.
Beat in the vanilla, then remove bowl from the saucepan (do not turn off the heat under the pan) and set aside at room temperature for 5 minutes.
Meanwhile, clean and dry the beaters again; place the egg whites, salt, and the remaining ½ cup sugar in another bowl and fit it snugly over the saucepan with the simmering water.
Beat at medium speed for 1 minute, until frothy.
Add in the cream of tartar and continue beating at medium speed, scraping down the sides of the bowl occasionally with a rubber spatula, until smooth, shiny, and satiny, about 3 minutes.
Remove the bowl from the saucepan and continue beating off the heat until room temperature, about 1 more minute.
Fold about half of the egg white mixture into the egg yolk mixture with a rubber spatula until smooth, then fold in the remaining egg white mixture.
Fold in the whipped cream gently, just to incorporate it without losing much of the volume.
Take the prepared loaf pan out of the freezer and pour this mixture into it.
Spread the mixture gently to the corners with a rubber spatula.
Return to you freezer and chill until cold and thick, about 6-8 hours.
Once the semifreddo is set, cover the pan tightly with plastic wrap to ward off freezer odors, can be stored this way for up to 2 weeks.
To serve, turn the loaf pan upside down on a serving platter; hold the plastic wrap at the sides and lift off the pan, wiggling it from side to side.
Pull the plastic off the semifreddo and cut into slices about 1 inch thick.