Once the cake has cooled, freeze it for about 1 hour. It is easier to cut when it is firm.
Once firm, flip it onto a cutting board and peel the rubber mat off. Using a paint roller, coat the base of the cake with a thin layer of white chocolate.
Once it sets, turn the cake over and cut it into ovals measuring 10cm/4in wide by 17.5cm/7in long.
Keep the cakes frozen until you are ready to use them. Discard them after 1 month.