Soak the porcini mushrooms in twice their volume of warm water for about 10 minutes to reconstitute them. Squeeze dry and combine with the cultivated mushrooms.
Heat the butter and oil in a frying pan and sauté the mushrooms. Cook until the liquid has reduced and then purée in a food processor.
Preheat the oven to 160°C. Mix the mushrooms with the béchamel sauce, egg and parmesan, and season with salt and pepper. Mix well and pour into 6 lightly greased plastic dariole moulds (which are available from many supermarkets) or ceramic soufflé ramekins.
Fold a tea towel and place in the bottom of a deep baking tray. Arrange the dariole moulds in the tray and pour in enough warm water to come halfway up the sides of the moulds.
Bake in the oven for about 35 minutes, or until set. The sformato should be fairly firm to the touch.