Preheat the oven to 325 degrees F (160 degrees C).
Place Duck Leg (4) in a smallish baking pan, fitting them somewhat snuggly.
Sprinkle with Kosher Salt (1 tablespoon).
Cover legs with Butter (1 pound), breaking the sticks in half if necessary. Cover pan tightly with aluminum foil. Be sure to wrap tightly.
Bake for 2 1/2 hours.
Remove pan from oven. Let cool briefly, then remove foil to cool completely. Once cool enough to handle, transfer legs to a plate if serving immediately or within the next few hours or a storage container if serving on subsequent days.
Transfer fat to a storage container. Store in fridge indefinitely. When you make confit again, use this fat in place of butter.
If possible, bring the cooked duck legs to room temperature an hour before cooking.
If you feel like being fancy, use the heel of your knife to whack off the top half of the bone nub at the end of each duck leg. Also, trim off any fat extending up the bone.
Heat a cast iron skillet over medium heat. Once hot, add duck legs skin side down. Let them crisp undisturbed for 1-2 minutes.
Shake the pan to make sure the skin isn't sticking. If it is, use a spatula to gently separate the legs from the pan, being careful not to tear the skin. Continue crisping, repositioning the legs if necessary to ensure the skin is browning evenly, about 5-8 minutes total.
If the legs have been refrigerated, once the skin is nicely crisped, transfer pan to the oven for 7-10 minutes or until the meat is nicely heated through. If the legs have not been refrigerated, you probably can get away with just flipping the legs over and cooking for 1 minute skin side up. You don't want to cook the legs too much on the flesh side or the meat will get too tough.