Preheat Oven on the broil setting. Heat your seasoned cast-iron skillet on medium-high heat. Rinse and blot dry your salmon filets with a paper towel. With flesh side up, sprinkle on salt, onion powder, garlic powder, and pepper if using. Once the pan is heated, place butter in the pan. If butter is burning, then your pan is too hot and will need to be reduced to medium heat. The high temperature will cook your salmon way too fast and cause the crust to burn. Also, do not crowd the pan as this will cause your salmon to steam. Place salmon flesh side down on the hot pan for 3 minutes. After 3 minutes, use a fish spatula to flip the salmon skin side down and cook for another 3 minutes. After 3 minutes, place the cast iron skillet, with the salmon still on it, into the oven to broil for 2 minutes. Depending on the thickness of the salmon, you may need to cook it for a couple of minutes more. Check it periodically as fish tends to over cook fast! Cook until the desired internal temperature is reached. Rare Salmon: Less than 120 degrees F, Medium Salmon: 135 -140 degrees F. Remove from the hot skillet, plate, and if desired squeeze a little lemon juice on it or place a lemon wedge on top of the salmon and serve.