Sprinkle the salmon with salt and pepper.

Place skin side up and rub 1 1/2 tablespoons softened butter all over the skin of the salmon.

Transfer the salmon, buttered side up, to a sheet pan.

Chill in the fridge to firm the butter, about 20 minutes.

Preheat the broiler to high.

Place the salmon about 5 inches from the heat source and cook until the skin is crispy and the fish is cooked through, 7 to 8 minutes.

In a small saucepan over moderate heat, whisk together the remaining 1 1/2 tablespoons butter, lemon zest, lemon juice, mustard and tarragon leaves.

Divide the sauce between 2 plates and place 1 piece of salmon on top of each pool of sauce.