Preheat oven to 350°F (175°C). Spread pistachios in a single layer on a baking sheet and roast for about 12 minutes, stirring halfway through.
Let pistachios cool completely.
Optional: Rub cooled pistachios in a clean kitchen towel to loosen and remove skins.
Add pistachios to a high-speed blender or food processor. Blend, stopping to scrape down sides as needed, until smooth and creamy. This may take 5–12 minutes depending on equipment.
Add salt and vanilla bean paste if using. Blend once more until fully combined.
If butter is thicker than desired, blend in 1–2 teaspoons neutral oil or pistachio oil until smooth.