Place raisins in container and cover with rum, let them soak for at least 8 hours.
Prepare 2 sponge cakes the day before you plan to put cake together. Cut a thin layer off the top of one sponge cake and set it aside; we will use this to cap the cake at the end.
In both sponge cakes, cut a line close to the edges on the inside. Rip out the inside of the cake, forming a bowl. Rip cake insides into small pieces. Set aside.
Whip together 1 cup sugar and heavy whip cream. Make sure that the whip cream is very cold, so it outputs the cream firm.
Drain all liquid from raisins. Mix together pieces of sponge cake, raisins and about 70% of the cream (keep the rest for outside of the cake).
Fill in bowls of sponge cake with mixture. Place them on top of each other, in between, you can fill it with a bit of cream.
Cover the top with the cake lid. Using the remaining cream, cover the cake. Decorate as you wish.