Prepare the Swiss roll:
Preheat the oven to 180°C. Separate the egg whites from the yolks. Whisk the yolks with the sugar until the mixture whitens. Stir in the cocoa powder, our Céliane baking mix , and the cornstarch. Beat the egg whites with a pinch of salt until stiff, then gently fold them into the mixture. Spread the batter on a baking sheet lined with parchment paper. Bake for 10 to 12 minutes.

Roll the biscuit:
As soon as it comes out of the oven, unmold the biscuit onto a clean, slightly damp cloth. Remove the parchment paper and immediately roll it up with the cloth. Let it cool.

Prepare the filling:
Whip the liquid cream until stiff. Gently fold in the chestnut cream, then add the chocolate chips.

Assemble the log:
Unroll the biscuit and spread the filling evenly. Roll the biscuit up tightly again and place it in the refrigerator for at least 2 hours.

Decorate:
Melt the dark chocolate in a bain-marie with a tablespoon of oil if desired. Spread it over the entire surface of the log with a spatula for a smooth or textured effect according to your taste. Sprinkle lightly with icing sugar for a snowy effect. Let the chocolate harden for a few minutes in the refrigerator before cutting into generous slices.