Pat salmon fillets dry on both sides with a paper towel. Rub all over with oil, then season with salt and pepper to taste. Sprinkle 1 teaspoon of Cajun seasoning on top of the salmon fillets.
Cook:
Heat a pan over medium-high heat, place salmon fillets, skin side-up (if using with skin), and sear the tops for about 4 to 6 minutes, flip the salmon, and cook an additional 6 to 8 minutes, until the salmon flakes separate easily. Turn off the heat. Salmon cooking times will vary by the thickness of the salmon pieces.
Add the Cajun butter:
While the salmon cooks, combine the softened butter with the remaining 3 teaspoons of Cajun seasoning (1 tablespoon) in a small dish.
Place a generous dollop of Cajun butter on top of each salmon fillet, allow it to melt for 30 seconds and plate the salmon. Use the back of a spoon to spread Cajun butter over the top.