6 ounces bittersweet chocolate
1/4 cup butter, plus extra for buttering the pan
4 egg yolks
11/2 cups sugar, divided
11/2 teaspoons vanilla, divided
7 egg whites, divided
1/8 teaspoon salt
1/4 cup cocoa, sifted
3/4 cup toasted hazelnuts
1 cup whipping cream
1 tablespoon amaretto liqueur

Heat oven to 350 F. Combine the chocolate and butter in a metal bowl set over a pan of simmering water and heat until melted and just combined, stirring occasionally. Remove from heat and allow to cool until just warm to the touch but still melted.

While chocolate is cooling, beat the egg yolks until light and fluffy. Slowly drizzle 1/4 cup of the sugar into yolks and continue to beat until fluffy and airy. Beat in 1 teaspoon of the vanilla until combined.

Separately, beat 4 of the egg whites with the salt to soft peaks. Drizzle 1/4 cup of the sugar into whites and continue to beat to stiff peaks. Set aside.

Fold whipped egg whites gently into whipped yolks, then fold in melted chocolate and the cocoa.

Spread batter into a 10-by-15-inch baking pan which has been buttered, lined with parchment paper, and buttered again. Bake cake for 10 to 12 minutes, until set. Turn cake out onto a rack to cool.

Place cooled cake on a cutting board and cut three rectangles from cake, each the dimensions of a 9-inch loaf pan (these rectangles will form layers in final dessert). Set aside.

In a small saucepan, combine 1/2 cup of the sugar with 2 tablespoons water. Heat to boiling (dissolving sugar), then reduce heat to a simmer and cook until syrup turns golden brown, several minutes. Stir in the toasted hazelnuts, then pour onto a lightly buttered baking sheet and allow to cool. Break pralines into pieces and coarsely process in a food processor; you should have 11/2 cups crushed praline. Set aside.

Whip the cream to stiff peaks, then fold in the amaretto liqueur and remaining vanilla. Separately, whip the remaining 3 egg whites. Gradually beat in the remaining 1/2 cup sugar until stiff peaks form. Gently fold egg whites into whipped cream, along with 1 cup crushed praline.

Line the loaf pan with plastic wrap. Place 1 layer of cake snugly into bottom of pan. Evenly spread 1/3 of semifreddo base over top of cake, and top with a generous tablespoon of crushed pralines. Repeat with remaining layers, for a total of 3 layers. Sprinkle remaining praline evenly over top of cake. Freeze, wrapped in plastic, until ready to serve.

To unmold cake, dip base of frozen loaf pan quickly in a bowl of hot water to loosen cake. Remove cake (still wrapped in plastic) from mold. Gently unwrap cake. Cake can be served whole on a serving platter garnished with orange slices or kumquats, or can be sliced.

Makes 10 servings.