Rinse the rice: Place rice in a fine-mesh sieve and rinse under cold water until water runs clear.
Cook the rice: In a medium saucepan, add rice and 1 cup water. Cook over medium heat until rice is tender and water is absorbed.
Add milk: Pour in milk and gently simmer over medium-low heat. Stir often to prevent the milk from sticking.
Sweeten the pudding: Mix sugar and cornstarch in a small bowl. Slowly whisk into the milk and rice mixture. Continue cooking until the mixture thickens slightly.
Flavor it: Stir in vanilla extract and crushed mastic, if using. Taste and adjust sweetness.
Prepare to bake: Pour the pudding into oven-safe ramekins or a baking dish.
Bake: Preheat oven to 350°F (175°C) and bake for 25–30 minutes, until the tops are lightly golden.
Cool and serve: Let it cool slightly. Sprinkle cinnamon on top before serving. Serve warm or chilled.
