Whisk together 1/2 cup of milk with the cornstarch in a small bowl; set aside.
Bring the rice and water up to a boil in a medium-sized, heavy-bottomed saucepan over medium heat. Cover the saucepan, turn heat down to simmer, and cook 12 minutes, stirring occasionally.
While the rice cooks, combine the remaining milk and sugar in another medium saucepan over medium heat. Cook until the sugar is dissolved and the milk starts to steam, stirring occasionally. Turn off heat.
When the rice is cooked, add the warm milk, cover the saucepan, and bring to a simmer over low heat, stirring occasionally. Cook 5 minutes, stirring frequently. Stir in the cornstarch/milk mixture, bring to a boil, and boil uncovered 3 minutes, stirring constantly. Remove from heat and stir in the rose water and orange blossom water.
Pour into 6 individual serving cups, cool to room temperature, then refrigerate to chill.
Serve chilled, garnished with the pistachios.