Combine the marinade ingredients and whisk until the molasses is dissolved and the ingredients are thoroughly combined. Place the pork chops in a large, resealable plastic bag and pour in the marinade. Press the air out of the bag and seal tightly. Turn the bag to distribute the marinade, place in a bowl, and refrigerate for 2 to 12 hours.
Remove the chops from the bag and discard the marinade. Let the chops stand at room temperature for 15 to 30 minutes before grilling.
Prepare the grill for direct cooking over medium heat (350° to 450°F), and preheat a grill pan for about 10 minutes.
Cut each sweet potato lengthwise in half and then cut each half into ½-inch slices. Put the slices in a large bowl, add the oil, season with the salt and pepper, and turn to coat.
In a small bowl mash the butter and miso until it is thoroughly blended. Set aside.
Spread the sweet potatoes in a single layer on the grill pan and grill over direct medium heat, with the lid closed, until tender, 10 to 15 minutes, turning once or twice. At the same time, grill the chops on the cooking grates over direct medium heat until still slightly pink in the center, about 8 minutes, turning once. (Because of the sugar in the marinade, the grill marks will be quite dark.)
Transfer the sweet potatoes to a medium bowl, add the miso butter, and toss until the butter melts and the potatoes are evenly coated. Serve the chops warm with the sweet potatoes on the side.