Double Chocolate Hazelnut Marjolaine

Toast hazelnuts on rimmed baking sheet in 350°F (180°C) oven until fragrant, about 15 minutes. Place nuts on tea towel; rub briskly with towel to remove as much of the skins as possible. Let cool.

In food processor, finely grind hazelnuts with 2/3 cup (150 mL) of the sugar; transfer to bowl and stir in cornstarch.

In large bowl, beat egg whites with cream of tartar until soft peaks form; beat in remaining sugar, 1 tbsp (15 mL) at a time, until stiff peaks form. Fold in nut mixture in 3 additions. Using offset spatula, spread on parchment paper-lined 17- x 11-inch (45 x 29 cm) rimmed baking sheet. Bake in centre of 275°F (140°C) oven for 1-1/2 hours or until crisp and golden.

Loosen edges of meringue from pan. Using serrated knife, cut crosswise into 4 rectangles. Let cool on pan on rack.

Bittersweet Chocolate Mousse: Place chocolate in bowl. In small saucepan, or measuring cup in microwave, bring cream to boil. Pour over chocolate; whisk until smooth. Refrigerate until cold but not firm, about 45 minutes.

Milk Chocolate Mousse: Place chocolate in bowl. In small saucepan, or measuring cup in microwave, bring cream to boil. Pour over chocolate; whisk until smooth. Refrigerate until cold but not firm, about 30 minutes.

Beat bittersweet chocolate mousse and milk chocolate mousse separately just until thickness of icing; set aside.

Garnish: Whip cream with icing sugar and hazelnut liqueur; set aside.

Place 1 of the meringues on platter; spread with half of the bittersweet chocolate mousse. Spread second meringue with all of the milk chocolate mousse; place on first meringue. Spread remaining bittersweet chocolate mousse over third meringue; place on top of milk chocolate mousse. Top with remaining meringue.

Spoon1/2 cup (125 mL) of the whipped cream into piping bag fitted with small star tip; set aside. Spread remaining cream over sides and top, leaving ends bare.

Press sliced hazelnuts into sides of cake. Pipe 16 rosettes along each long side on top. Place whole hazelnut in every other rosette. Refrigerate for 24 hours. Let stand at room temperature for 45 minutes before slicing. (Make-ahead: Cover with plastic wrap and refrigerate for up to 48 hours.)