Start by preparing the dark chocolate mousse. Separate the eggs, then break the dark chocolate into pieces and melt it gently in a bain-marie over low heat. Add the butter in pieces and stir until smooth. Incorporate the egg yolks one by one, whisking constantly, then stir in the amber rum from Martinique.
Beat the egg whites to stiff peaks, add the caster sugar, and gently fold them into the chocolate mixture. Refrigerate the mousse.
Prepare the puff pastry sheets: Divide the dough into 3 equal parts. Roll out one portion into a 3 mm thick square and cut into 6 rectangles (10 cm x 5 cm). Place on a moistened baking tray, refrigerate for 1 hour.
Preheat the oven to 210°C (th. 7). Bake for 30 minutes, then cool on a wire rack. Repeat with the remaining dough.
For the white chocolate sauce: Heat the cream in a saucepan, add white chocolate pieces, and stir over low heat until melted and smooth.
Assemble on dessert plates: Place a pastry rectangle, top with mousse using a spatula or piping bag, add another rectangle, more mousse, and finish with the top rectangle. Decorate with chocolate shavings and drizzle with sauce.
