Slice the sponge cake and spread with the jam. Sandwich the slices together and slice into 1 inch thick cubes. Spread the sponges into a trifle bowl, sprinkle generously with the sherry. Reserve a few berries for garnish, and add the berries to the mixture. Break the chocolate bar into small pieces and throw ½ over the sponge. Prepare the custard. Heat the milk with the zests until it’s almost boiling. In a large bowl, whisk the yolks together with the corn flour, sugar and vanilla. Pour the hot milk over the egg mixture, whisking all the time. Return to a clean pot and gently heat the custard until it begins to thicken. Remove from the heat and stir in the remaining chocolate, set aside to cool slightly. Pour the custard over the sponge mixture and spread evenly to cover. Chill in the fridge until set. Decorate with the whipped cream and garnish with the berries toasted almonds and the remaining chocolate.