Slowly add scalded milk to egg mixture and stir well. Pour mixture into casserole. Bake in a pan filled with hot water which comes halfway up the sides of the casserole for about 1 hour. Spoon into 4 dessert dishes. Mix together flour, walnuts, Brown Sugar Twin, and margarine.
Press remainder firmly in pan.
Bake for 12 minutes. Meanwhile, beat cream cheese, egg, Sugar Twin, lemon juice, milk, and vanilla until smooth. Spread over baked crust, spreading to cover. Sprinkle with remaining crumb mixture. Return to oven and bake 25 minutes.