In a small saucepan, heat up only 1/2 cup of cream until boiling. Remove from heat. Add gelatin and stir until completely dissolved.
Pour the cream and gelatin mixture over white chocolate. Let sit for 20 to 30 seconds, then whisk until smooth. Set aside to cool to room temperature. Stir occasionally. It should slightly thicken.
Please read the instructions before proceeding. It is important to plan well when making the cake. You cannot speed up any process. Every layer has to set. Be gentle when folding the whipped cream into chocolate mixtures. Whisk gently. It is important to fold in until no streaks remain for the proper setting of the mousse. Take your time - this cake is worth it.