1) Reserve some of the berries whole, for a garnish. Take the remaining berries, crush about one-quarter of them, slice the rest, and combine with the jam. Allow the mixture to sit for an hour or so.
2) Sprinkle the cubes of cake with the syrup.
3) Layer the cake, pastry cream, and blackberries and/or jam in six small bowls or trifle dishes.
4) Refrigerate for several hours. Top with softly whipped cream; garnish with berries or a swirl of jam, and serve.
Yield: 6 individual trifles.