Prep the Oven: Preheat your oven to 400°F (200°C). Grease your 10x15 inch jelly roll pan and line it with parchment paper, leaving a slight overhang.
Whip the Eggs: In a large bowl, beat egg yolks and 1/2 cup sugar until pale and thick (ribbon stage). In a separate clean bowl, whip egg whites and remaining 1/4 cup sugar to stiff peaks.
Fold the Batter: Sift cocoa powder into the yolk mixture and fold gently. Add 1/3 of the egg whites to lighten the batter, then carefully fold in the remaining whites to preserve air bubbles.
Bake: Pour batter into the pan and smooth carefully. Bake for 10-12 minutes until the cake springs back when touched. Do not overbake.
The Towel Roll: Dust a clean kitchen towel with cocoa powder. Immediately invert the hot cake onto the towel and peel off the parchment. Starting at the short end, roll the cake up with the towel inside. Cool completely (approx 60 mins).
Make Filling: Whip the heavy cream, mascarpone, powdered sugar, and vanilla bean paste until stiff peaks form.
Assemble: Unroll the cooled cake. Spread filling evenly, leaving a 1-inch border. Re-roll the cake without the towel. Chill for 20 mins.
Decorate: Melt ganache ingredients together. Let cool slightly to thicken. Spread over the log and use a fork to create "bark" lines. Dust with powdered sugar "snow" before serving.