Place potatoes on a sheet of parchment-lined foil, and season with salt and pepper. Fold and crimp edges to seal, leaving an opening at the top. Pour in 1/2 cup beer (you'll have some left over). Crimp remaining edges to seal. Cook pack set over a campfire or on a medium-high grill (or nestled near coals) until potatoes are tender, 20 to 25 minutes. Top potatoes with butter.