1. Prepare the Vanilla Layer:

1. In a large mixing bowl, whip the heavy cream to soft peaks. Set aside in the refrigerator.
2. In a heatproof bowl, whisk the egg yolks and sugar together until pale and creamy. Place the bowl over a pot of simmering water (double boiler) and whisk constantly for about 5 minutes, or until the mixture thickens slightly.
3. Remove from heat and stir in the vanilla extract. Allow to cool.
4. Gently fold the whipped cream into the egg mixture until smooth.

2. Prepare the Chocolate Layer:

1. Repeat the steps for the vanilla layer, but after whisking the egg yolks and sugar, stir in the melted chocolate and sifted cocoa powder. Let cool before folding in the whipped cream.

3. Assemble the Zebra Semifreddo:

1. Line a loaf pan with plastic wrap, leaving enough overhang to cover the top later.
2. Pour a small layer of vanilla mixture into the pan, smoothing it out with a spatula.
3. Add a layer of chocolate mixture on top. Continue alternating layers until both mixtures are used up.
4. Tap the pan gently on the counter to remove air bubbles. Fold the plastic wrap over the top to seal.

4. Freeze and Serve:

1. Freeze the semifreddo for at least 6 hours, or overnight, until firm.
2. To serve, invert the pan onto a serving plate and remove the plastic wrap. Slice with a sharp, warm knife to reveal the zebra stripes.
3. Garnish with chocolate shavings, powdered sugar, or fresh berries, if desired.