Set a bowl of ice water near the stove. Put the egg yolks and sugar in a bowl. With a whisk or electric mixer, beat until thick and pale yellow, 1 to 2 minutes. Slowly add the hot milk. Pour the mixture into a 1- to 2-qt. saucepan and cook over medium heat, stirring constantly, until it's thick enough to coat a metal spoon and reaches about 165° on an instant-read thermometer, 4 to 5 minutes. Do not let custard boil or it will curdle. Remove from heat and set in bowl of ice water; stir often until cold. Stir in 1 cup nougat and the liqueur.
Line a 5- by 9-in. loaf pan with a single sheet of plastic wrap, leaving overhang on all sides. Sprinkle the bottom first with remaining 1 cup nougat, then the 1/4 cup chocolate, spreading them evenly.
In a bowl with clean beaters or a whisk, whip egg whites to stiff peaks. In a separate bowl, whip 1 1/4 cups cream to firm peaks.
Pour custard over egg whites and fold in gently. Fold in whipped cream until no white streaks remain.
Pour the custard mixture into the loaf pan. Cover with overhanging plastic wrap. Freeze semifreddo until firm, at least 6 hours or up to 1 week.
For the chocolate sauce: In a small saucepan, bring remaining 1/2 cup cream to a simmer. Put the 1/4 lb. chocolate in a bowl and pour hot cream over it. Let stand until chocolate is softened, about 3 minutes, then whisk until smooth.
Remove plastic wrap covering loaf pan. Put a platter upside down over pan. Invert and remove pan and plastic wrap. If you want to remove wrinkle marks, smooth them over using a spatula. Cut the semifreddo into 1/2-in. slices and place on dessert plates. Spoon chocolate sauce over and around it. Serve immediately.
Note: Nutritional analysis is per serving with 1 Tbsp. sauce.