Coat salmon filet with melted butter. Rub in the chili powder and sprinkle on the Cajun Seasoning. Heat a cast iron skillet over medium high heat. Add olive oil and heat until smoking. Turn the heat down a little and add the salmon filet, skin side down and cook until half done. With a spatula, slide the meat off the skin, remove skin from skillet and flip the salmon filet over and cook until tender and flaky.
For the mayonnaise. In a small bowl combine all ingredients and mix well.
Brush the bread slices with butter and broil until toasty. Smear both sides of bread with the mayonnaise, divide the salmon filet in half, top each sandwich with a leaf of butter lettuce.