Place the thawed raspberries and strawberries in a large sieve and drain over a bowl. Place the fruit in a medium bowl and set it aside. Measure the syrup. You should have 1 cup; if necessary, add enough cranberry juice to bring up the level.
Transfer 1/4 cup of the syrup to a small saucepan. Add the cornstarch and stir to mix well. Stir in the remaining syrup and mix well. Bring to a boil over medium-high heat. Cook, stirring, until the juice thickens and turns clear. Stir in the brandy or rum (if using). Measure out and reserve 1/2 cup of the fruit for garnish.
Meanwhile, prepare the custard, cover, and chill.
Cut the cake into bite-size pieces using a serrated knife. Place half of the pieces in a large straight-sided glass bowl. Spoon half of the fruit over the cake. Cover with half of the custard. Repeat to use all the cake, fruit, and custard.
Garnish the top with the reserved fruit and the fresh raspberries or strawberries. Cover and refrigerate for at least 2 hours and up to 6 hours before serving. --