Butter the bottom of a 9-inch oven safe tart, flan or pie dish. Arrange blackberries evenly over butter. Using an electric mixer, combine egg yolks, whole egg and sugar. Beat until light and creamy. Add cream, flour, lemon juice and pinch of salt, and blend well. Pour mixture over blackberries and bake at 350 degrees for about 30 minutes or until golden brown on top and toothpick comes up clean. When cooled, sprinkle lightly with powdered sugar. Serve alone, with whipped cream or vanilla ice cream.