A sleek Franco-American roulade is a chocolate cake, lavishly filled with a glistening puree of Armagnac-soaked prunes and a rich Gianduja mousse.
Line an 11-by-17-inch baking pan with parchment paper, and set aside. Place the egg yolks in the bowl of an electric mixer fitted with the whisk attachment. Beat on high speed until pale yellow, 4 to 5 minutes. Transfer to a medium bowl, and set aside. Wash and dry mixer bowl.
Heat oven to 400 degrees. Place egg whites in mixer bowl, and beat on medium speed until soft peaks form, 1 to 2 minutes. Increase speed to medium high, and whip until stiff peaks form while gradually adding the sugar. Transfer egg-white mixture to a large bowl.
Using a rubber spatula, fold egg-yolk mixture into egg-white mixture. Sift cocoa powder over top; fold in. Pour batter into pan; smooth top with an offset spatula.
Bake until cake springs back when touched, 7 to 8 minutes. Remove pan from oven, and immediately turn out onto a wire rack lined with parchment paper. Peel parchment paper from top of cake, and cool completely.
Lay a clean piece of parchment paper on top of cake, and invert. Peel off and discard old parchment. Using an offset spatula, spread prune-Armagnac filling over cake. Spread Gianduja mousse evenly over filling.
Starting at a shorter end of cake, roll cake into a roulade. Wrap in parchment; secure with tape. Freeze until set, about 2 hours.
When ready to serve, remove parchment. Place roulade on wire rack set over a baking pan. Pour chocolate glaze over cake; tap pan on countertop, smoothing surface. Chill until set, about 15 minutes.
Transfer to a serving platter; garnish with hazelnuts and sugared prunes. Serve remaining chocolate glaze on the side, if desired.