Prep. time: 30 minutes
Cooking time: 8 to 10 minutes
Refrigeration: 12 hours

1.  Preheat the oven to 200° C / 400° F.
2. Line a baking sheet with a sheet of buttered parchment paper.
3. Separate the eggs.
4. In a bowl, whisk the egg yolks and powdered sugar (preferably with an electric hand mixer) until light and pale, then sprinkle in the flour, folding it in without overworking. In a separate bowl, beat the egg whites with a pinch of salt to stiff peaks.
5. Melt the butter. Add half the egg whites and the melted butter to the batter and fold in gently, then fold in the remaining beaten whites. 
6. Spread the batter into the prepared pan, smoothing out the surface. Place on the middle rack of the oven and bake for 8-10 minutes, depending on thickness.
7. Remove from the oven and turn the sponge out onto a tea towel and remove the parchment paper. To remove it easily, brush the paper lightly with water. After a few minutes, the paper will come off on its own.

Filling

1. Soak the sheets of gelatin in a bowl of cold water to soften.
2. Gently whisk together the mascarpone and chestnut cream.
3. Melt the drained gelatin in the microwave or in a small saucepan over low heat.
4. Whip the cream with the icing sugar until thick. Pour the melted gelatin over the cream, whisking constantly, until well combined. 
5. Add to the mascarpone-chestnut mixture, folding in with a rubber spatula. 

Assembly

1. Cut the sponge into 4 more or less equal rectangles.
2. Rinse a 27 x 10 cm loaf pan with water. Without drying it, line it with plastic wrap.
3. Lay in one piece of sponge, a layer of cream, another layer of sponge, a layer of cream, and so on, ending with a layer of cake. 
4. Cover with plastic wrap and refrigerate for at least 6 hours.

Icing

1. Pour the cream into a saucepan and bring to a boil. Pour over the chopped chocolate in a bowl.
2. Combine well and add the butter. Mix again until smooth. Let cool. 

Finishing

1. Unmold the log on a serving platter, cover with the chocolate icing and refrigerate another 6 hours.
2. Before serving, decorate the top of the log with some candied chestnuts and sprinkle some pieces of candied chestnut all around.