Preheat the oven to 450 degrees F (230 degrees C). Brush 2 small gratin dishes with 1 tablespoon melted butter.

Whisk crème fraîche, white wine, lemon zest, cayenne pepper, and kosher salt together in a bowl until smooth; add scallops and toss to coat. Let sit until flavors begin to meld, about 5 minutes.

Divide scallops between prepared gratin dishes; pour remaining crème fraîche sauce over top. Top with tarragon and Parmesan cheese; drizzle remaining 1 tablespoon butter over top.

Bake in the preheated oven for 4 minutes. Increase oven setting to broil; continue cooking until scallops golden brown, slightly springy, and opaque, 2 to 4 minutes more.