Place rack in center of oven and heat to 350 degrees F.
Butter a 11 by 17-inch jelly roll pan or 12 by 17 1/2-inch sheet pan and line with parchment.
Heat 1 cup cream to a simmer in a small saucepan. 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.
Whip egg whites and 2 tablespoons sugar to stiff glossy peaks, about 1 1/2 minutes. Whisk 1/3 of egg whites into chocolate mixture. Pour this lightened mixture into egg whites and fold in gently, until smooth and well combined.
Pour batter into 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 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 with cocoa powder.
For 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 to make a tight spiral. Gently peel parchment off as cake layer rolls away. Complete the roll keeping it just on far edge of parchment. Cover roll by tucking loose parchment around and underneath so the cylinder 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, 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; 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.
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