Preheat your oven to 400°F (200°C).
In a small bowl, mix together the Cajun seasoning, garlic powder, onion powder, smoked paprika, salt, black pepper, and cayenne pepper (if using).
Pat the chicken breasts dry with paper towels. Rub the spice mixture evenly over both sides of the chicken breasts.
In an oven-safe skillet, heat the olive oil over medium-high heat. Once hot, add the chicken breasts and sear for about 4-5 minutes on each side until golden brown.
Add the butter to the skillet, allowing it to melt and baste the chicken for an additional minute.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the chicken reaches an internal temperature of 165°F (75°C).
Remove the skillet from the oven and let the chicken rest for 5 minutes before slicing. Garnish with chopped green onions before serving.