Stracciatella Tortoni Cake with Espresso Fudge Sauce
For the Crust • ⅔ cup finely ground amaretti (Italian almond macaroons; about 17; use a food processor)
• ¼ sliced almonds with skin, toasted, cooled, and finely ground (in processor) • 3 tablespoons unsalted butter, melted and cooled
For Tortoni Filling • 3 large egg whites, at room temperature 30 minutes • ¼ cup sugar • ½ teaspoon cream of tartar • 1 ¼ cups chilled heavy cream • 2 tablespoons Disaronno Amaretto or other almond-flavored liqueur • 3 ½ oz fine-quality bittersweet chocolate (no more than 60% cacao if marked), shaved with a vegetable peeler • ¼ cup sliced almonds with skin, toasted and cooled
For Sauce • ⅓ cup heavy cream • 3 tablespoons light corn syrup • 3 tablespoons packed dark brown sugar • 2 tablespoons instant-espresso powder • 2 tablespoons unsweetened cocoa powder • 3 ½ oz fine-quality bittersweet chocolate (no more than 60% cacao if marked), finely chopped • ½ teaspoon pure vanilla extract
Butter a 9x5-inch loaf pan and line bottom and short sides with a strip of parchment paper, leaving 4 inches of overhang on each end.
Stir together ground cookies, ground almonds, and butter, then firmly press over bottom of pan. Freeze until firm, about 30 minutes.
Make Tortoni Filling:
Beat egg whites with sugar, cream of tartar, and ⅛ tsp salt in a large metal bowl set over a large saucepan of simmering water using a handheld mixer at medium-high speed until whites hold soft peaks and an instant-read thermometer registers 170°F, about 7 minutes.
Remove bowl from pan and continue to beat meringue until it just holds stiff peaks, about 2 minutes.
Beat cream with Amaretto in another bowl at medium speed using cleaned beaters until it just holds stiff peaks. Fold in half of meringue gently but thoroughly. Fold in remaining meringue along with chocolate. Spoon over crust, smoothing top with offset spatula. Sprinkle with almonds. Freeze, uncovered, until firm, about 3 hours.
Bring cream, corn syrup, brown sugar, espresso powder, cocoa, ⅛ tsp salt, and half of chopped chocolate to a boil in a small heavy saucepan over medium heat, stirring until chocolate is melted. Reduce heat and cook at a slow boil, stirring occasionally, 5 minutes. Remove from heat. Stir in vanilla and remaining chocolate until smooth. Cool to warm.
Dip bottom of loaf pan in 1 inch warm water in a roasting pan 10 seconds, then lift tortoni out of pan using parchment paper. Transfer to a platter. Peel paper from tortoni.
Let stand 5 minutes to soften slightly. Cut into 6 triangular wedges. Thin sauce with additional cream if necessary and serve with tortoni.
Tortoni can be frozen up to 3 days (wrap in plastic wrap after 3 hours).
Sauce can be made 1 week ahead and chilled, covered. Reheat before using.