Prepare the Pistachios: Soak pistachios for 30 minutes (optional for smoother texture) and blend until finely ground.
Blend the Mixture: In a blender, combine the ground pistachios, coconut milk, sweetened condensed milk, evaporated milk, and vanilla extract. Blend until smooth and creamy.
Add Spices and Alcohol (Optional): Add cinnamon, nutmeg, and salt. If using rum, add it now and blend again to combine.
Taste and Adjust: Taste and adjust sweetness or spice level to your preference.
Chill: Pour the mixture into an airtight container and refrigerate for at least 2-3 hours to allow the flavors to meld.
Serve: Shake well before serving. Pour into glasses over crushed ice, if desired. Garnish with a sprinkle of cinnamon or crushed pistachios.