Rinse the rice with cold water.
In a medium saucepan over medium heat, combine the rice and 2 cups of water.
Let the mixture boil, then reduce heat, cover the saucepan and simmer until the water is absorbed (about 5-10 minutes).
Add in the milk, sugar, and the mastic (if using) to the rice. Cover and simmer over low to medium heat by stirring occasionally.
In a small bowl, mix the cornstarch and 2 tbsp of water.
Add in the cornstarch mixture to the rice pudding. Cover and let it simmer for another 5 minutes over low heat by stirring regularly until it reaches a thick and moist consistency.
Turn off heat and stir in the rosewater.
Scoop the pudding into individual small bowls. Add toppings as desired (ideas for toppings can be found below).
Cool at room temperature.
The rice pudding can be served warm or cold. Once the pudding is cooled, cover with plastic wrap and store in the fridge to be used at a later stage.