Preheat oven to 350°F / 180°C and spread your pistachios out on a baking sheet.
Roast the pistachios for 7 minutes, giving them a little shake half way through to turn them over a bit. Do not overcook the pistachios or they will be bitter and brown. When you start to smell them, they are ready.
Remove from the oven and let the nuts cool slightly.
Once cooled, put the pistachios in a food processor and blend them. You'll see the pistachios go through stages, from chunky, to a fine powder, then it will start to clump together, eventually form a ball, then finally it will turn into a lovely, smooth pistachio paste.
If the nuts seem too dry and are clumping together and don't look like they will blend up, you can add a tablespoon of neutral oil to help rehydrate them.
Notes
What kind of pistachios to use:
Shelled: Use pre-shelled pistachios to save time and sore fingers!
Unsalted: Best for baking, so you can control the salt. Add salt later if needed.
Raw or Roasted: Both work, but raw pistachios are less likely to be dried out.