Chocolate and boysenberry crème fraiche tart

1 sheetSweet shortcrust pastry, defrosted1Egg, beaten, for brushing350 gDark chocolate, good quality, 70 per cent cocoa (Main)250 gCreme fraiche, at room temperature1 canBoysenberries, 400g, drained (reserve the syrup) (Main)2 punnetsFresh mixed berries, e.g. raspberries, boysenberries, blackberries (Main)