Using three different flat containers divide cornstarch, egg and panko keeping each separate.
Season the fish with salt and freshly ground black pepper. Dip the skin side of the fillets in cornstarch, then egg wash, then in panko to create a nice crust.
Add oil to a saute pan large enough for both fillets and place over medium-low heat. Add fillets crust side down and cook for 5-7 minutes until the panko begins to dehydrate a bit.
Add in the cubes of butter to the pan and transfer to the oven and cook for 5 more minutes until the fish is opaque, tender and warmed through.