Continue blending. The natural oils will release and the mixture will begin to clump together.
Stop and scrape down the sides as needed.
Blend until completely smooth and creamy, with no clumps remaining.
Transfer to a 4 oz airtight jar.
Refrigerate for extended shelf life.
Notes
You can use more pistachios if you need a larger batch. Blend longer for extra smooth pistachio butter, or leave slightly textured for a thicker consistency.