Place the seeds in a baking or cookie sheet and toast in a a 350°F oven for 20-25 minutes, moving them around often, until golden and fragrant.
Be careful to not take the seeds too far. Sunflower seeds are very delicate and will burn fairly easily. You want them to be a very light golden color, no more.
Let the seeds cool for a few minutes then transfer them to the bowl of your food processor
Process for a total of 25-30 minutes, stopping every 2 to 3 minutes or so to break down the clumped-up mixture and scrape the sides a bit.
After about 20 minutes, the seeds should start releasing their oil and the butter will finally relax and become creamy. From this point, you will want to let it run for 5 to 10 minutes until really smooth and creamy.