Make the miso butter: in a mixing bowl, combine the softened butter, white miso and brown sugar; mix until smooth and fully combined. Set aside.
Heat a large nonstick frying pan or skillet over medium-high heat and add the olive oil, swirling to coat the pan.
Place the salmon fillets presentation side down in the hot pan (top of the fillet touching the pan). Sear for 3 minutes without moving.
Flip the fillets and cook the underside for 2 minutes.
After 2 minutes on the second side, spoon the miso butter over the top of each fillet and allow it to melt for about 1 minute.
Sprinkle with sesame seeds and sliced green onion, then serve immediately.