Serves 6-8
 
5 egg yolks plus 2 whole eggs
½ cup granulated sugar
¼ tsp. salt
2 tsp. pure vanilla extract
2 cups heavy whipping cream
1/3 cup powdered sugar
½ cup sour cream or plain yogurt
4 oz. semi-sweet chocolate, chips or pieces
 
Topping:
4 oz. semi-sweet chocolate, chips or pieces
1/8-1/4 cup heavy whipping cream
¼ cup roasted nuts or mini chocolate chips
 
Line a loaf pan (9x5 inch x 3 inch) with plastic wrap, including up all sides of pan.
 
Custard: 
Prepare a double boiler by bringing water to a boil in a small saucepan. Reduce to a simmer and place a heat proof bowl the sauce pan.  Make sure that the water does not come in contact with the bowl.
Place eggs, yolks, sugar and salt in the bowl. With a whisk or hand held mixer beat mixture until pale and thick, about 5 minutes. Remove from heat and whisk in vanilla extract. Set aside to cool.
In a bowl of a stand mixer using the whisk attachment, beat the heavy whipping cream and powdered sugar until it begins to thicken.  Add sour cream or yogurt and continue to beat until thick and fluffy.
Combine 1/3 of the whipped cream into the egg custard mixture to lighten it up with a whisk. Fold in the remaining whipped cream until well combined. There shouldn’t be any white streaks in the mixture.  If needed, whisk to a smooth it out, but try not to overwork the mixture so that it does not deflate. Pour half of the cream mixture in the pan. 
Place the melted chocolate in a plastic bag with the tip snipped off.  Drizzle half of the chocolate over the cream.
Pour the remaining cream over the chocolate and swirl the remaining chocolate on top of the cream. 
Use a butter knife to create a swirl pattern. Let the knife go all the way through the pan down to the bottom so that the chocolate in the center gets distributed evenly. Cover lightly with plastic.
Place the loaf pan in the freezer and let set overnight or for at least 6 hours. 
 
Topping: 
Combine chocolate with the cream and melt either on the top of a double boiler or in a microwave.  Whisk until smooth and shiny. If chocolate is too thick add more hot cream and whisk.  
Take the frozen semifreddo out of the freezer and invert onto a platter. Discard the plastic wrap and using a fork drizzle the melted chocolate mixture over the ice cream and garnish with nuts or chocolate chips. 
 
This tastes best slightly softened at room temperature for about 10 minutes.