Roast the pumpkin seeds in the over at 165ºC (fan assisted) for 10 minutes and then leave till warm-to-the-touch *
Blend them in a high-speed processor/blender. It's best to do so at intervals so the seeds (and machine) don't overheat. The longer you blend the runnier the consistency will be. For a smooth pumpkin seed butter, blend for about 20 minutes with breaks every 3 minutes, or so.
Once the homemade pumpkin seed butter is ready, transfer it to an airtight glass container and keep this in a cupboard for a few weeks or the fridge for 2-3 months. It can also be frozen, for longer
* If they're slightly warm when blending then this should help them blend quicker.