Chefs Julia Child and Jacques Pepin shared their recipe for this mouthwatering cake, a log-shaped cake made from a souffle-like mixture.
Preheat oven to 350 degrees with rack in center. Butter an 11-by-17-inch jelly-roll pan or a 12-by-17 1/2-inch sheet pan, and line with parchment paper.
In a small saucepan, heat 1 cup cream to a simmer. Add chocolate, reduce heat, and whisk until chocolate is melted. As soon as mixture is a uniform dark color, remove from heat and let cool for a few minutes.
In a large bowl and using a hand mixer, whip egg whites and 2 tablespoons sugar to stiff, glossy peaks, about 1 1/2 minutes. Whisk one-quarter of the egg-white mixture into the chocolate mixture. Gently fold chocolate mixture back into the original egg-white mixture, and mix until smooth and well combined.
Pour batter into the prepared pan, and spread it in an even layer with a rubber spatula. Bake until cake is set and puffy, 10 to 12 minutes. Transfer to a wire rack, and cool to room temperature. Lift parchment paper to remove cake from pan, and set it on work surface with long side facing edge of table. Using a fine-mesh sieve, lightly dust cake with cocoa powder.
Make the creme chantilly: Whip the remaining 1 cup cream with the remaining 1 1/2 tablespoons sugar, the vanilla, and cognac. Spread evenly over entire surface of cake.
Roll the cake lengthwise, starting at a point 2 to 3 inches over the creme chantilly. Roll cake another few inches, pressing against parchment paper to make a tight spiral. Gently peel parchment paper off as cake layer rolls away. Complete the roll, stopping at the far edge of the parchment paper. Tuck the loose parchment paper around and underneath the cylinder so it is well wrapped and can be moved easily. Serve immediately, or refrigerate for up to 4 hours.
When ready to serve, transfer roulade to serving platter. Remove parchment paper, gently rolling cake into center of platter, with seam on bottom. (If roll has slumped or twisted, lay a piece of plastic over top and sides, and reshape with hands.) With a sharp knife, trim both ends of roll crosswise or on a diagonal. Dust top with confectioners' sugar and cocoa powder, and garnish with seasonal fruit. To serve, cut the roll into 1-inch-thick slices, and lay flat on dessert plates; top with additional creme chantilly if desired.