Whisk eggs, yolks, vanilla, and sugar in an electric mixer bowl to combine.

Place the bowl over a double boiler.

Heat the mixture until sugar dissolves and it reaches 160 degrees Fahrenheit on a thermometer.

Transfer the mixture back to the mixer.

Whip on medium-high speed until pale and very thick, for 4-5 minutes.

Pour the mixture into a 13x9 casserole dish.

Transfer the dish to the freezer.

Freeze for 2-4 hours until set completely.

Gently fold the mixture 2-3 times while freezing.

When almost frozen, generously dollop jam around the semifreddo.

Use a butter knife to swirl the jam into the semifreddo.

Freeze until ready to serve.