Preheat the oven to 375 degrees F.
In a small bowl, combine 3 cloves garlic (minced), 2 tablespoons fresh dill (chopped), 1 teaspoon salt, 1/2 teaspoon freshly ground black pepper, 3 tablespoons freshly squeezed lemon juice, and 1/4 cup extra virgin olive oil; stir to make the marinade.
Place the salmon fillets in a glass baking dish and pour the marinade over them, turning the fillets to coat. Cover and refrigerate for at least 1 hour (preferably several hours), turning occasionally.
Remove the salmon from the refrigerator. Divide the 2 tablespoons butter into 4 equal pieces. Tear or cut four sheets of aluminum foil large enough to wrap each fillet.
Place one marinated fillet in the center of each foil sheet. Spoon some of the marinade from the baking dish over each fillet, place one butter piece on top of each fillet, then fold and seal the foil to form individual packets. Set the sealed packets in a baking dish or on a rimmed baking sheet to catch any drips.
Bake the sealed packets in the preheated oven for 30–40 minutes, depending on the thickness of the fillets, until the salmon is opaque and flakes easily with a fork.
Carefully open the foil packets (watch for hot steam) and serve the salmon immediately.