Heat butter in a cast iron skillet and add the salmon filets skin side down. Cook the salmon for 4-5 minutes till the skin is slightly crispy.
Whisk together orange juice, zest, mustard, soy sauce, honey, vinegar and salt in a small bowl and add it to the pan. Bring this to a quick boil and simmer the sauce for 2-3 minutes till it thickens up. Gently flip the salmon and coat it with sauce (I like to spoon the sauce all over). Top with sesame seeds and serve immediately.