For each fillet, have a separate foil packet. Grease the foil with the butter, place the fillets on it, and brush them with the butter as well, coating both the sides. Dab some lemon juice over the fillets, sprinkle the whiskey, and season them with the salt and pepper. Bring the folds of the foil together to make a packet. Bake the salmon in a 300 °F preheated oven for 10-15 minutes. Meanwhile, whisk the dill, mayonnaise, leftover lemon juice, and salt and pepper together. Serve the fillets hot with the dill-mayonnaise sauce poured over them.