Cut the cakes into about 1 1/2cm (half inch) slices.
Butter a bowl (or mold or springform pan) and place 1 1/2cm slices of jam roll around the sides, fitting them tightly together so that there are no spaces for the custard to seep through. Reserve some slices for placing on top of the Bavarian cream (which will be the bottom of the cake when removed from the mold). Refrigerate until you are ready to fill the mold.
If using gelatine, soak gelatine in 3 tablespoons cold water.
Heat milk with vanilla over a medium heat. Add gelatine (or agar-agar) to the milk and bring it gently to the boil, stirring continuously. Remove from the heat and allow to cool slightly.
While cooling, whisk together egg yolks, icing sugar and salt until smooth, and pour the yolk mixture into the milk while stirring constantly.
Stir continuously with whisk over a gentle heat until the mixture coats the back of a spoon. The mixture must NOT boil. Pour into a bowl and allow to cool, then refrigerate until the custard is cold and just beginning to thicken.
While the custard is cooling, whip the cream until it just starts to thicken, then add the icing sugar. Continue whipping to soft peaks. Fold in the fruit or purée and/or liqueur if using.
Fold cream together with the cooled, but not set, custard. (If the custard has cooled and set a bit too fast, use your mixer to make it creamy before blending it with the whipped cream.).
Pour into the prepared dish, top with reserved slices of sponge roll and chill overnight.
To unmold, briefly place the bowl into warm water, cover with serving plate and invert. Tap gently and ease the bowl off of the cake. Add whipped cream and fruit as a garnish if you like.
For vegetarian use agar-agar.
