8 (3-inch-long) russet potatoes (about 2 1/4 pounds), scrubbed and thoroughly dried, 2 tablespoons unsalted butter (melted), Kosher salt, Freshly ground black pepper, 4 to 6 ounces thinly sliced smoked salmon, 3 tablespoons crème fraîche or sour cream, 4 teaspoons black caviar.
Heat the oven to 200°C and arrange a rack in the middle. Pierce each potato several times with a fork or sharp knife. Place the potatoes directly on the oven rack and bake until the skins are crisp and a knife easily pierces the potatoes, about 50 minutes. Transfer to a wire rack until cool enough to handle, about 10 minutes. Set the oven to the broil setting. Slice each potato in half lengthwise. Using a spoon, scoop out the flesh, leaving about 1/4 inch intact; reserve the flesh for another use. Brush the insides of the potatoes with the melted butter and season with salt and pepper. Flip the potatoes over, brush the skin sides with butter, and season with salt and pepper. Evenly space the potato halves skin-side up on a baking sheet and broil until the butter foams and the skins start to crisp, about 2 to 3 minutes (keep a close watch so they don’t burn). Flip the potato halves over and broil until the top edges just start to brown, about 2 to 3 minutes more.
Divide the salmon evenly among the skins and top each with about 1/2 teaspoon of the crème fraîche and 1/4 teaspoon of the black caviar. Serve immediately.