Slice potatoes and add to large bowl of water. Clean potatoes and drain. Pat dry.

Lay two sheets of aluminum foil and overlap the edges, spray with cooking spray.

Arrange potatoes in two layers.

Season and add butter pats on top of potatoes.

Fold foil over potatoes and seal completely.

Prepare grill (medium-high heat).

Place potato packet on grill and cook until heated through and sizzling, turning occasionally, about 25 minutes.

Transfer packet to serving plate and pierce foil with fork to release steam.

Open packet carefully.