Prepare the creamy filling: Heat the heavy cream in a small saucepan and remove it from the heat just before it boils. Pour it over the white chocolate pieces in a bowl. Stir to completely melt the chocolate. Then, incorporate the Vinsobres biscuit powder.
For the chocolate covering, heat the heavy cream and, before it boils, remove it from the heat and pour it over the chocolate pieces in a bowl. Mix until the mixture is smooth.
Chill the Vinsobres biscuit cream and the ganache.
Preheat the oven to 180°C. Grease a baking sheet of about 26*32 cm and line it with parchment paper.
Separate the egg yolks from the egg whites. Beat the yolks with the sugar until you have a thick and creamy mixture. In another bowl, add the milk, then the flour, baking powder, and cocoa powder. Mix well. Beat the egg whites until stiff peaks form, then fold them into the batter using a spatula.
Pour the batter onto the baking sheet to form a uniform rectangle and bake for about ten minutes.
Meanwhile, prepare the syrup: mix the sugar with the amber rum and heated water until the sugar is completely dissolved.
Dampen a kitchen towel and place it on the work surface. Invert the baked cake onto it. Remove the baking sheet and the top parchment paper. Brush the sponge cake with syrup and then add the Vinsobres biscuit cream. Gently roll up the cake.
Cover the log with the chocolate ganache. Refrigerate for at least 2 hours before serving.