1. Spread the pistachios on a baking sheet.
2. Bake in the preheated oven for 4 minutes at 180ºC/350ºF.
3. After this time, let the pistachios cool at room temperature.
4. Transfer the pistachios to a food processor or blender.
5. In 30-second intervals and with a lot of patience, start grinding the pistachios until the butter forms.
6. Store refrigerated in a container with a lid.