Bring 2/3 cup water to a boil; stir in couscous and salt. Remove from heat; cover and let stand 5 minutes. Add milk and sugar; bring to a boil. Reduce heat to medium-low; stir until mixture is slightly thick. Transfer to a bowl; whisk in egg. Return to saucepan; cook, stirring frequently, until creamy (about 5 minutes). Transfer to a clean bowl, and stir in vanilla. Let pudding cool to room temperature, stirring occasionally. Spoon into 4 bowls, and chill, loosely covered, for at least 1 hour. Drizzle with maple syrup before serving.