Empty the beverage carton and wash it out thoroughly. Cut the upper third of the carton away. Line the inside with two strips of baking parchment that cross over on the bottom of the box.
Melt the dark chocolate over a water bath. Using a pastry brush, completely paint the inside of the box with the chocolate.
Put the milk in a pan and melt in the white chocolate while stirring. Add the gelatin; as soon as it's dissolved transfer the milk mixture to a bowl. Once it reaches room temperature, fold in the cream. Refrigerate the mousse. After that, carefully fold in the raspberries.
Spoon the mousse into the beverage carton until it reaches the top, then turn it out onto a plate. Carefully let the chocolate cube slip from the carton and remove the baking parchment.