Cook potatoes 25 to 30 minutes over medium heat, covered, until tender. Drain and combine potatoes with sugar, milk, egg, melted butter, and vanilla. Stir until mixed through. Break up potatoes, but don't mash. Pour mixture into greased 2 quart casserole.
Meanwhile, combine brown sugar and flour. Cut in cubed butter until mixture resembles corse crumbs. Stir in pecan pieces and sprinkle mixture on top of potatoes. Bake uncovered 25 minutes or until set at 350.