Preheat the oven to 180°C (350°F). Line a 10×15 inch jelly roll pan with parchment paper and lightly grease the paper.
In a large bowl, whisk the egg yolks with 50 grams of granulated sugar until the mixture is pale and thick. Stir in the vanilla extract.
In a separate bowl, sift together the cocoa powder, flour, and salt.
Gently fold the dry ingredients into the egg yolk mixture until just combined.
In a clean bowl, beat the egg whites until soft peaks form. Gradually add the remaining 50 grams of granulated sugar and continue beating until stiff peaks form.
Carefully fold one-third of the egg whites into the chocolate batter to lighten it, then gently fold in the remaining egg whites until no white streaks remain.
Fold the melted butter into the batter gently.
Pour the batter evenly into the prepared pan and spread it out smoothly.
Bake for 10 to 12 minutes, or until the cake springs back lightly when touched.
While the cake is baking, lay a clean kitchen towel on a flat surface and dust it generously with powdered sugar.
Once the cake is done, immediately invert it onto the sugared towel. Carefully peel off the parchment paper.
Starting from the short edge, roll the cake and towel together into a tight spiral. Let it cool completely on a wire rack.
To prepare the filling, heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Remove from heat and pour over the chopped chocolate in a heatproof bowl.
Let sit for 2 minutes, then stir until smooth and glossy. Allow the ganache to cool until thickened but still spreadable.
Unroll the cooled cake carefully and spread the chocolate ganache evenly over the surface.
Roll the cake back up without the towel. Place seam side down on a serving plate.
Dust the top with additional powdered sugar before serving.