Place the raisins in a bowl and cover them with rum. Allow them to soak at room temperature for at least 8 hours.
Bake two sponge cakes (preferably a day in advance). Cut a thin layer off the top of one of the sponge cakes and set it aside.
Cut alongside the inside edges of each cake layer and rip out the insides of the cake to form a bowl. Rip up the insides of the cake into small pieces and set them aside.
Using an electric mixer, whip 1 cup of sugar with the extra cold heavy whipping cream.
Strain the rum out from the raisins. Mix the pieces of sponge cake, raisins, and about 70% of the cake cream together.
Fill the sponge cake bowls with the rum raisins cream mixture. Place the cake layers on top of one another.
Cover the top of the cake with the sponge cake lid. Using the remaining cake cream, frost the outside of the cake and decorate it as you wish. Enjoy!