- 6 ounces fine-quality 60%-cacao bittersweet chocolate, finely chopped
- 1/4 cup water
- 5 large eggs, separated, at room temperature 30 minutes
- 1/4 teaspoon salt
- 1/2 cup sugar, divided
- 1 tablespoon unsweetened cocoa powder
For meringue layers:
- 2/3 cup hazelnuts (3 1/2 ounces)
- 3 large egg whites at room temperature 30 minutes
- 1/8 teaspoon salt
- 1/8 teaspoon cream of tartar
- 1/2 cup sugar
Line second sheet pan with parchment paper.
Melt chocolate with water, then cool to lukewarm.
Beat yolks, salt, and 1/4 cup sugar in a large bowl with an electric mixer at high speed until thick and pale, about 5 minutes with a stand mixer or 8 minutes with a handheld. Fold in melted chocolate.
Beat whites with cleaned beaters until they hold soft peaks. Beat in remaining 1/4 cup sugar, 1 tablespoon at a time, and beat until whites just hold stiff peaks, about 5 minutes.
Fold one third of whites into chocolate mixture to lighten, then fold in remainder gently but thoroughly. Spread batter evenly in lined sheet pan.
Bake until puffed and a wooden pick inserted into center comes out with a few crumbs adhering, 25 to 30 minutes. Transfer pan to a rack, then cover top of soufflé with 2 layers of damp paper towels. Let stand 5 minutes. Remove towels and cool soufflé completely in pan (soufflé will deflate as it cools). Sift cocoa over soufflé, then loosen edges with a sharp knife.
Halve soufflécrosswise, cutting through parchment, to form 2 (10-by 7 1/2-inch) layers.
Toast hazelnuts, then cool, wrapped in a kitchen towel, and rub off any loose skins. Reduce oven to 250°F.
Finely chop nuts. Beat whites with salt and cream of tartar using electric mixer until they just hold soft peaks. Beat in sugar, 1 tablespoon at a time, and beat until meringue is stiff but still glossy.
Line bottom of third sheet pan with parchment. Put small dabs of meringue under corners of parchment to secure to baking sheets.
Fold nuts into meringue and spread evenly in pan. Bake until set and pale golden, 25 to 30 minutes.
Halve meringue crosswise, cutting through parchment, to form 2 (10-by 7 1/2-inch) layers. Return to oven and bake until crisp, 45 minutes to 1 hour more. Cool in pan, then peel off parchment.