1Prepare grill for cooking over medium-hot charcoal (moderate heat for gas).
2Season salmon all over with salt and pepper, then grill, flesh sides down, on lightly oiled grill rack (covered only if using gas grill) 4 minutes. Turn fillets over and grill (covered only if using gas grill) until just cooked through, 4 to 6 minutes more. Sprinkle fillets with zest and top each with 1 tablespoon lime butter sauce.
3Cooks' Note: If you aren't able to grill outdoors, salmon can be cooked in a hot lightly oiled well-seasoned large (2-burner) ridged grill pan over moderately high heat.