Line a 9x5x3-inch loaf pan with plastic wrap. For crust:In a small bowl, combine crushed amaretti and melted butter. Press mixture onto the bottom of the prepared pan; set aside.
In a large bowl, combine chocolate-hazelnut spread and Frangelico; set aside.
In another large bowl, beat whipping cream with an electric mixer on medium to high speed until soft peaks form (tips curl). Gradually fold the whipped cream into chocolate mixture.. Gently fold in the 3/4 c pistachio nuts.
Spoon mixture over crust, spreading evenly. Cover and freeze for 8 to 24 hrs.
To serve, sift cocoa powder onto a serving platter. Invert semifreddo onto platter; remove plastic wrap. If desired, sprinkle with additional pistachio nuts. Use a serrated knife to slice semifreddo. Makes 12 servings
Nutritional Facts per serving: 381 cal, 28g fat, 50mg chol, 67mg sodium, 29g carbs, 1 g fiber, 5 g protein.