Preheat oven to 350°. Butter baking dish. Heat olive oil in heavy skillet over high heat. Sauté mushrooms 2 minutes until liquid evaporates. Season with sea salt and freshly ground black pepper. Remove from skillet. Set aside and combine with brioche. Place shallots in skillet. Cook over medium heat without browning. Deglaze pan with white port. Reduce by half. Pour in cream. Bring to boil. Remove from heat. Set aside. Let cool. In separate mixing bowl beat eggs until well combined. Pour in cream, herbs and porcini powder. Season with sea salt and freshly ground black pepper. Place mushrooms and brioche into baking dish. Pour egg and cream mixture over top. It is very important to ensure that all of the bread is saturated with the custard. Cover tightly with foil. Bake at 350° 30 minutes or until set.