Soften the butter until very soft. In a mixing bowl, combine the 1/4 cup softened butter, 1/4 cup white miso, and 1 tablespoon brown sugar; mix until smooth and evenly combined. Set the miso butter aside.
Heat a large nonstick frying pan or skillet over medium-high heat and add 1 tablespoon olive oil, swirling to coat the surface.
Place the 4 salmon fillets presentation side down (the top of the fillet facing the pan) in the hot pan. Sear without moving for 3 minutes to develop a crust.
Flip the fillets and cook the underside for 2 minutes.
After those 2 minutes, spoon the miso butter over each fillet and let it melt over the salmon for about 1 minute, spooning the melted butter over the tops as it melts.
Sprinkle 1 teaspoon sesame seeds and the sliced green onion over the glazed salmon, then remove from the pan and serve immediately.