In a large, heavy-bottomed pot, bring the rice, milk, and cardamom to a boil over medium-high heat, stirring constantly with a heat-safe spoonula to help keep the milk from burning.
Reduce the heat so that the milk is gently simmering and cook for 45 to 50 minutes, stirring often. The rice should be tender and the milk will have reduced by half, giving a porridge-like consistency.
Add the sugar, rosewater or vanilla, and pistachios. Stir and turn off the heat. Serve either warm or chilled, garnished with extra pistachios. Goes well with fresh fruit too.