Heat small heavy nonstick skillet on medium-low heat. Meanwhile, partially cut wiener lengthwise in half, being careful to not all the way through to opposite side of wiener.
Add wiener, cut sides down, to skillet; cook 1 min. on each side or until lightly browned on both sides.
Place bun, cut sides up, on work surface. Place 2 Singles pieces on bottom half of bun; top with wiener and remaining Singles pieces. Fold over top of bun to enclose filling. Spread outside of sandwich with butter.
Cook 3 min. on each side or until Singles are melted and sandwich is golden brown on both sides.