Pare top one-third of apples. Place each apple on square of aluminum foil that is large enough to completely wrap the apple. Fill centers with raisin mixture. Top each with one-fourth of the butter. Wrap foil around apple, pinching firmly together at top.
Place apples on rack or in basket.
Close cover securely. Place pressure regulator on vent pipe. Cool cooker at once. Pour into eight 6-ounce custard cups. Cover each securely with aluminum foil.