In another saucepan, combine 1 1/2 cups cooked rice, 1 1/2 cups milk, sugar and salt. Cook over medium heat until thick and creamy, 15 to 20 minutes. Stir in remaining 1/2 cup milk, beaten egg and raisins (if using). Cook 2 minutes more, stirring constantly. Remove from heat, and stir in butter and vanilla. Serve warm.