Spray a loaf pan with nonstick cooking spray and then line it with plastic wrap, allowing the excess to hang over the ends and sides.
Place 4 ounces of amaretti cookies in the bowl of a food processor and finely grind. Add 3 tablespoons melted unsalted butter and process until the crumbs are moistened. Press the crumb mixture onto the bottom of the prepared loaf pan. Refrigerate.
Whisk 1/2 sup sugar, 8 egg yolks, 1/3 cup espresso, 2 tablespoons dry marsala and a pinch of salt in a large metal bowl. Set the bowl over a saucepan of simmering water (do not allow the bottom of the bowel to touch the water). Whisk the egg mixture until it is thick and creamy and a thermometer inserted into the custard registers 160 degrees F. This process takes around 5 minutes. Set the custard into another bowl of ice to cool completely.
Using an electric mixture, beat 1 cup double (heavy) cream and 1/4 cup of sugar until firm peaks form.
Using a large rubber spatula gently fold the whipped cream into the custard. Spoon the mixture onto the prepared crust.
Fold the overhanging plastic wrap over the custard and freeze until frozen, at least 8 hours and up to 3 days. When ready to serve, unfold the plastic wrap, invert the semifreddo onto a platter and peel off the plastic wrap. Cut the semifreddo into 1-inch or so slices and serve. This will serve 8 people.