1. Heat the oil in a heavy skillet and add the annatto seeds. Stand back well away from the pot or use a spatter screen, as the seeds may pop. Swirl the oil in the pot and fry the seeds for 1 to 2 minutes, until all their color is released into the oil. Strain out the seeds and pour the oil into a heatproof bowl. Cool completely, then aside.
2. Place the flour in a bowl with the salt and cut in the butter pieces with a pastry blender, until it reaches the consistency of coarse meal with pea-size pieces. (Alternatively, place the flour, salt, and butter in the bowl of a food processor fitted with a plastic blade and pulse quickly.) Do not overwork the dough. If using a food processor, remove the dough from the machine at this point and place in a mixing bowl. Using a spoon, stir the dough while adding drops of ice water, until the dough just comes together in a ball.
3. Wrap the dough in plastic wrap and flatten to form a disk. Chill in the refrigerator for at least 2 hours, or as long as overnight. The dough may be frozen and then thawed in the refrigerator for later use.