Preheat oven to 325 °F (165 °C). Spread pistachios in an even layer on a baking sheet and toast for about 5–8 minutes, until fragrant but not browned

Remove from oven and let the pistachios cool slightly, about 5–10 minutes, until just warm

Transfer warm pistachios to the bowl of a food processor. Process continuously, scraping down the bowl every few minutes as the nuts progress: first into a fine crumb, then a thick paste, and finally a smooth butter (about 5–10 minutes total)

If the mixture seems dry or thick, add the optional oil (1 tsp at a time) and continue blending until you reach your desired consistency

Add a pinch of salt, pulse briefly to incorporate, then taste and adjust seasoning if needed

Transfer the pistachio butter to a clean jar and let cool completely. Seal with an airtight lid.

Store at room temperature for up to 1 week, or refrigerate for up to 4 weeks; stir before use if oil separates