Place the pistachios in a food processor and blend until smooth, scraping down the processor as you go. It will take about 20 minutes for the oils to release and turn into a thick paste. Be patient!
Once a paste has formed, add the salt and vanilla bean paste, blending until well combined. Transfer to a well sealed jar.
Pistachio Panna Cotta
In a small bowl, gently stir together the gelatin powder and water. Let it sit for at least 5 minutes until a gel forms.
In a medium saucepan over medium-low heat, whisk together the heavy cream, milk, ½ cup (120 g) pistachio butter, and sugar until smooth. Bring to a simmer or until the edges begin to foam and bubble.
Once simmering, turn off the heat and whisk in the vanilla and gelatin until dissolved.
Divide into 6 serving glasses and cool to room temperature, about 45 minutes to 1 hour.
Place the glasses in the fridge and chill for at least 2 hours or overnight. Cover them with a layer of plastic wrap to prevent any fridge odors from affecting the taste.
When ready to serve, top with chopped pistachios and dried rose petals (optional).
Notes
The pistachio butter makes about 1 cup (240 g). Store the other half in a jar and keep in the fridge for up to 1 month. Use it as you would any other nut butter. You should also have enough left over to make another batch of panna cottas.
Unmolding the panna cotta: Dip the cup halfway in a bowl of warm water for about 30 seconds, just enough to loosen the custard from the cup. Dry off the outside then place a serving plate on top and flip it over. If it doesn't come loose, use the edge of a thin knife to loosen it or give it a few shakes (while holding the plate) until it plops right out.