Prepare a high-heat fire, and let it burn for 30 minutes.
Meanwhile, to toast the hazelnuts, place a cast-iron skillet over a medium-high flame. Add the hazelnuts and cook, shaking the pan occaisionally, until the nuts are golden and the skin has begun to crackle and flake. Let them cool completely before chopping.
Place the mushrooms in the center of a large sheet of foil. Dot the mushrooms with the butter and sprinkle on the chives, salt, and pepper. Fold the edges of the foil securely over the mushrooms, forming a packet, and place the packet on the grill grate. Cook until the mushrooms are tender and turning golden, about 15 minutes. Open the packet and continue roasting until the juices evaporate, up to 10 minutes more.
Transfer to a bowl and toss in the toasted hazelnuts to serve.