Start by making a berry jam. Place the frozen berries and sugar into a pan and cook down together for 10-15 minutes and allow to cool down completely. Remove half of the berries and blitz till smooth. Next, whip the double cream with the vanilla then whisk in the cold custard to form a Chantilly cream. Place half of the cream into another bowl and mix in the pureed berry mixture. You will now have 2 Chantilly creams, 1 vanilla and one berry flavoured. To put the dessert together, cut the sponge into 8 disks the size of your glasses. Place a disc of sponge into the base of each glass and spoon over the remaining berry jam. Spoon over the vanilla flavoured Chantilly cream and cover with another disc of sponge. Next, spoon over the berry flavoured Chantilly cream and top with fresh berries. Place into the fridge to chill for at least an hour before serving.