When the plancha is hot, place the salmon fillets on it, skin side down. The butter is most likely melted at this point, so remove it and keep it warm. Close the grill and cook the salmon fillets until they are opaque through, about 8 minutes; if they are almost but not quite opaque, remember they will continue to cook once off the grill. Slip a sturdy metal spatula between the meat and the skin of each fillet, and transfer the meat to a cutting board with a trough around it to catch the juices or a large plate or platter. Carefully remove the skin from the plancha— it will be golden and crisp— and reserve. Season the salmon with salt and pepper and let it sit for a few minutes so that the fillets give up their juices.