Heat the oil and annatto seeds in a small skillet over medium heat until the seeds start to give off a lively, steady sizzle. Don’t overheat the mixture or the seeds will turn black and the oil a nasty green. Once they’re sizzling away, pull the pan from the heat and let it stand until the sizzling stops.
Strain as much of the oil as you are going to use right away; store the rest for up to 4 days at room temperature in a jar with a tight-fitting lid.
Yields about a cup