Pat the mahi mahi fillets completely dry with paper towels to ensure optimal searing.
In a small bowl, combine salt, black pepper, garlic powder, and smoked paprika. Season all sides of the fillets evenly.
Heat the olive oil in a heavy-bottomed skillet over medium-high heat until it reaches its shimmering point.
Carefully place the fillets in the skillet. Sear undisturbed for 3 to 4 minutes until a golden-brown crust has formed and the fish releases easily from the pan.
Flip the fillets. Add the butter to the skillet and allow it to foam.
Continue cooking for another 3 to 4 minutes, frequently spooning the melted butter over the fish, until the internal temperature reaches 145 degrees Fahrenheit (63 degrees Celsius).
Remove the skillet from heat. Deglaze the pan slightly with lemon juice over the fish and garnish with chopped parsley before serving.