Slip these mussels into a 500° oven to bake if you don’t have a broiler.
Put 2 pounds cleaned mussels and ½ cup white wine into a medium pot, cover, and steam them open over high heat, about 5 minutes. Set aside to cool completely.
Twist off and discard one of the shells from each mussel (discard any unopened mussels), making sure that the remaining shell has a mussel in it. Reserve the mussel broth for another use.
Put 4 tablespoons softened butter, 1 handful chopped flat-leaf parsley leaves, 1 small clove minced garlic, 4 pinches pimentón and kosher salt to taste into a bowl and combine into a paste. Slather each mussel in its shell with some of the seasoned butter and arrange on a shallow baking or broiler pan. Refrigerate the mussels until the butter is hard.
Preheat the broiler. Broil the mussels until the butter is bubbling hot, about 2 minutes.