Preheat oven to 375 degrees. Butter the bottom and sides of a 9-inch pie plate; set aside. In a medium bowl, combine whipping cream, milk, eggs, flour, granulated sugar if desired, melted butter, vanilla and salt. Using an electric mixer on low speed, beat until smooth. Arrange mixed berries in prepared pie plate or custard cups. Pour batter over berries. Bake for 50 to 55 minutes or until puffed and light brown. Cool for 15 to 20 minutes on a wire rack. Sift powdered sugar over top. Serve warm.