Pour 1/2 cup of heavy cream into a small flat plate or bowl and place in the freezer for about an hour until frozen solid.
Combine 1/3 cup of milk with the arrow root in a small mixing bowl. Stir until completely smooth and set aside.
Using a mortar and pestle, grind the saffron threads to a fine powder and set aside.
Combine the remaining milk and heavy cream, sugar, mastic powder, salt, and vanilla in a large saucepan and bring to a gentle simmer over medium-high heat, about 10 minutes. Be sure to stir the mixture frequently to avoid any milk crusting on the bottom.
Reduce the flame to low when the milk starts to steam, but well before it comes to a boil.
Add the ground saffron and rosewater to the saucepan and stir. The milk mixture should turn a vibrant golden color and become quite aromatic.
Stir the arrow root and milk mixture one more time and add it to the saucepan. Continue to stir gently over medium heat for about 4-5 minutes or until the mixture has thickened slightly.
Remove from the heat, transfer to a medium-sized shallow bowl and allow to cool to room temperature. Cover and refrigerate for 2 hours, or overnight.
Transfer the chilled milk mixture into an ice cream maker, and process it according to the manufacturer’s instructions.
Remove the previously frozen heavy cream from the freezer, break it into roughly 1/2 inch pieces and add them along with the chopped pistachios to the final stage of the ice cream maker’s churning process. The ice cream will have a soft and creamy texture at this stage and is ready to be served. Alternatively, it can be returned to the freezer for 2 hours for a firmer texture.
Transfer the ice cream to an air-tight and freezer-safe container, cover with parchment paper to prevent icicles, and freeze for long-term storage.