Grind the saffron threads with a small pinch of granulated sugar using a mortar and pestle until finely ground. Transfer to a small bowl, add the hot water and leave at room temperature until the ice has completely melted and the saffron has released its colour and aroma.
In a small bowl, thoroughly combine the salep powder with 150g of the granulated sugar. Mixing the salep with the sugar before adding it to the milk helps prevent clumping.
Pour the milk into a large saucepan and add the remaining 150g granulated sugar. Heat over medium-low heat, stirring regularly, until the sugar has completely dissolved and the milk is hot but not boiling.
Reduce the heat to low. Whisking continuously, gradually sprinkle the salep and sugar mixture into the warm milk. Add it slowly rather than all at once to help it incorporate smoothly.
Continue cooking over low heat for 10–15 minutes, stirring constantly, until the mixture has thickened and developed a smooth, slightly elastic consistency. Do not allow it to boil vigorously.
Remove the saucepan from the heat. Stir in the bloomed saffron and rosewater until evenly incorporated.
Allow the ice cream base to cool completely to room temperature. Cover and refrigerate for at least 4 hours, or preferably overnight, until thoroughly chilled.
Pour the chilled mixture into an ice cream maker and churn according to the manufacturer's instructions, usually for 20–30 minutes, until thick, creamy and approximately the consistency of soft serve.
Gently fold the frozen cream pieces and pistachios through the freshly churned ice cream, working quickly so the cream pieces remain frozen.
Transfer the ice cream to an airtight freezer-safe container. Press a piece of parchment paper directly onto the surface, cover and freeze for at least 4 hours, or until firm.
Remove the Bastani Sonnati from the freezer 5–10 minutes before serving to soften slightly. Scoop into bowls and garnish with additional pistachios, if desired, or serve as ice-cream sandwiches