For the semifreddo, line a loaf pan with plastic wrap, leaving a 3-inch overhang. In a medium heatproof bowl, whisk the egg yolks, sugar and 1/3 cup water to blend. Set the bowl over a large saucepan of simmering water and whisk until thickened and an instant-read thermometer registers 160 degrees , about 3 minutes. Remove from the heat and set the bowl over a large bowl of ice water. Using an electric mixer, beat until cool, 1 to 3 minutes.
In a large bowl, using an electric mixer, beat 3/4 cup cream until soft peaks form. Gently fold the whipped cream, then the chopped nougat (or cookies and nuts) into the egg-yolk mixture.
Gently transfer the mixture to the prepared loaf pan, taking care not to deflate it. Smooth the top. Fold the plastic wrap over to cover. Freeze until firm, at least 8 hours and up to 3 days.
For the sauce, in a small saucepan, bring the remaining 1/2 cup cream to a simmer over medium; add the chocolate and stir until melted and smooth.
Unwrap the semifreddo; invert onto a plate. Remove the loaf pan and gently peel off the plastic wrap. Cut crosswise into 8 slices. Place 1 slice on each plate, top with about 1 tbsp. sauce and serve immediately.