Preheat grill to medium-high, around 400°F. Whisk melted butter, olive oil, garlic powder, smoked paprika, salt, and pepper in a large bowl. Add sliced potatoes and toss to coat.

Lay four 12-inch sheets of heavy-duty foil on the counter. Divide potatoes among them, piling loosely in the center. Fold foil over and crimp edges tightly to form sealed packets.

Place packets on grill over direct heat, close lid, and cook 15 minutes. Carefully flip each packet and cook another 10–12 minutes, until potatoes are tender and edges are golden.

Transfer packets to plates. Open carefully and let the steam escape. Spoon any juices from the packet over the potatoes and serve immediately.