While the grill is heating up, trim the bottom 1-2 inches off the asparagus with a knife—this part is tough. You can save the ends to make vegetable stock.
Tear off a large sheet of heavy-duty aluminum foil.
Place the asparagus in the center of the foil. Season with salt and pepper to taste.
Bring the long sides of the foil together above the asparagus. Fold them down over each other 2–3 times to seal. Then fold in the ends tightly to create a sealed packet.
Place the packet on the grill. Cook the asparagus for 10-15 mintues until tender. You can check the asparagus with an instant read thermometer. It needs to reach 180 degrees.
Remove the asparagus from the grill with a pair of tongs. Carefully open up the packet (watch out for the steam!) and put into the butter. Close back up and allow to sit for 2-3 minutes to melt the butter and then serve.