Spread hazelnuts on rimmed baking sheet; roast in 325°F/160°C oven until slightly darkened and fragrant, 13 to 15 minutes. Immediately transfer to tea towel; rub with towel to remove skins.

Line 2 baking sheets with parchment paper; with tip of spoon or butter knife, draw an 8-inch/20 cm circle centred on each.

In food processor, pulse 3/4 cup/175 mL of the hazelnuts, 1/4 cup/60 mL of the granulated sugar and cornstarch until finely ground; whir in cocoa.

In bowl, beat egg whites with cream of tartar until soft peaks form; gradually beat in remaining granulated sugar until stiff glossy peaks form. Fold in hazelnut mixture. Evenly spread half on each prepared baking sheet to fill circles.

Bake in top and bottom thirds of 275°F/135°C convection or 300°F/150°C conventional oven, switching and rotating pans halfway through, until tops are firm, 50 to 70 minutes.

While warm, slide long metal spatula under meringues to loosen; carefully transfer to rack and let cool completely. (Make-ahead: Store at room temperature in airtight container for up to 3 days.)

Chocolate Mousse Cream: Whip cream; refrigerate, covered. In heatproof bowl over hot (not boiling) water, melt dark chocolate with butter, stirring lightly. Remove from heat; set aside.

In separate heatproof bowl over hot (not boiling) water, whisk egg yolks until warmed. Remove from heat; beat until thickened and light, about 5 minutes. Whisk into chocolate mixture. (If chocolate seizes or separates, refrigerate bowl, covered, until fat is set. Scrape into blender; blend until smooth, adding a splash whipping cream.)

In separate bowl, beat egg whites and icing sugar until stiff glossy peaks form. Stir a third of the egg white mixture into chocolate mixture until blended; fold in remaining whites. Let cool. Fold in whipped cream. Refrigerate until chilled, about 2 hours.

In heatproof bowl over hot (not boiling) water, melt milk chocolate. Stir in remaining 1/3 cup/75 mL of the hazelnuts until coated. With fork, transfer to rack or waxed paper; let cool until set.

Remove Chocolate Mousse Cream from refrigerator; let stand for 20 minutes to soften.

Place 1 meringue on serving platter and the other on flat baking sheet. Pipe or gently spread half of the Chocolate Mousse Cream over top of each meringue. Carefully ease meringue from baking sheet, cream side up, onto meringue on platter. Sprinkle with icing sugar. Top with chocolate-covered hazelnuts. Serve immediately.

Makes 10 to 12 servings.