To prepare the chocolate filling:
Heat the liquid cream in a saucepan over low heat.
Add the chocolate pieces to the hot cream and stir until completely melted and smooth.
Remove from the heat, add the butter and mix until you have a smooth ganache. Leave to cool to room temperature.
To roll the cake:
As soon as it comes out of the oven, turn the sponge out onto a clean, damp tea towel. Carefully remove the baking paper.
Immediately roll up the sponge cake with the cloth, without filling, and let cool like this. This allows the cake to keep its rolled shape without cracking.
Assembling the roll:
