Prepare two sheets of parchment paper sized to your oven tray and butter them. Place one on the tray.
Separate the egg whites from the yolks. Beat the yolks with sugar until pale and creamy.
Incorporate the flour and yeast, then fold in the melted chewable chocolate and 125g butter (melted over very low heat). Whip the egg whites to stiff peaks and gently fold them in. The batter should be firm enough not to spread excessively during baking.
Pour the batter onto the prepared tray in the center and spread it to within 3-4 cm of the edges.
Bake quickly at 240°C (th. 7), watching closely to avoid burning or drying out.
Once baked, remove from the oven and immediately cover with the second buttered parchment sheet (buttered side down). Roll the paper and biscuit together while hot and let cool completely.
Unroll carefully once cool, unmold, and remove the papers. Handle gently as it's more delicate than a standard roll.
In a warmed bowl, cream 80g butter until smooth. Mix in the chestnut cream and flavor with kirsch. Blend thoroughly.
Spread half the chestnut cream over the unrolled biscuit. Re-roll carefully. Coat the exterior with the remaining cream, smoothing with a spatula. Chill for several hours to firm up. Trim the ends at an angle with a sharp knife.
Simple Option: Coarsely grate chocolate with a paring knife under firm pressure to create shavings. Sprinkle generously over the cake.
Elegant Option: Melt the fondant chocolate in a bain-marie (add a bit of butter if desired). Pour onto a cool marble slab or baking sheet and chill. Scrape thin curls using a hot, dry knife blade, as with butter curls. Arrange delicately on the cake.
Source: Les bons desserts by Claire Mignieres, Baudouin.
