Combine brown sugar, smoked paprika, dried oregano, dried thyme, garlic powder, onion powder, cayenne pepper, salt, and pepper in a medium mixing bowl. Mix well.

Coat the salmon fillets with the spice mixture, pressing firmly to adhere.

Let the seasoned fish rest while preparing the skillet.

Preheat a large cast-iron skillet over medium-high heat with butter until melted and sizzling.

Place the salmon fillets skin-side down and cook for 3-4 minutes until a blackened crust forms. Flip and cook for another 3-4 minutes until done.

In a mixing bowl, combine diced mango, chopped red onion, lime juice, and cilantro. Mix gently.

Let the salmon rest for a minute then plate it and spoon salsa over the top.