Lay out 4 large pieces of heavy duty foil and spray well with cooking spray.
Slice the potatoes into ¼ inch slices and place into the center of each piece of foil.
Season with salt and pepper and top with garlic butter.
Place a chicken breast on top of the potatoes and generously brush each side of each chicken breast with barbecue sauce. Wrap each foil packet, folding the edges to seal tightly.
Place foil packets on the grill, potato-side down. Cook for 25-30 minutes, flipping halfway through, until the potatoes are tender and chicken reaches 165°F in the thickest part.
Notes
Leftovers will keep in the refrigerator for 4 days.