Passion fruit is brightly colored yellow, sweet-tart tropical fruit that is quite nice with vanilla custard, panna cotta or a lemon mousse. The fruit is pulpy and full of seeds, however, so we like to use a passion fruit purée concentrate, like the one available at Perfect Purée, for this preparation, which is more syrup than sauce. The cognac is just a flavoring agent so mix it in after the syrup is prepared and cooled.