In a small pot, bring cold water and sugar to the boil. Keep boiling for a few minutes, stir constantly until the sugar dissolves completely. Remove the pan from the heat and let cool down. When the syrup is cold, you may add vanilla or the Triple Sec liqueur (optional).

Preheat oven to 380 degrees F (180°C). Line a baking sheet with parchment paper.

Using an electric mixer, beat eggs and sugar until the mixture becomes thick, fluffy, and triples in volume.

Using a spatula, gently fold in flour and cocoa powder.

Spread the batter evenly in thin layer into the prepared baking sheet. Bake for 7 to 8 minutes in the preheated oven until a toothpick inserted in center of biscuit comes out clean.

Turn upside down onto clean kitchen towel and carefully remove parchment paper. Let cool down.

Boil the heavy whipping cream. Pour the boiling cream over the dark chocolate. Wait for one minute.

Then mix it until the mixture is smooth.

Add butter and mix until this is fully dissolved.

Let cool the ganache for about 30 minutes in the fridge. Mix and check frequently: the ganache is ready when it’s not anymore liquid and has a spreadable texture.

Beat together very cold heavy cream and cold mascarpone cheese until this is whipped. Keep gradually adding sugar.

Paint the biscuit with sugar syrup.

Spread the chantilly cream evenly over the biscuit.

Roll up firmly.

Tightly cover with plastic wrap, and keep in the fridge until chocolate ganache is ready.

Remove the plastic wrap. Spread the ganache over the cake. Using a fork make lines to look like a log. Transfer the cake to a serving platter and let stand in the fridge for a night or at least 3 hours.

Before serving, dust with cacao cocoa powder and decorate with meringues or pieces of chocolate.