A gathering becomes instantly festive when you offer these delicate hors d'oeuvres topped with caviar.
Place potatoes in a large saucepan, and cover with cold water. Bring to a boil; reduce to a simmer, and cook until just tender when pierced with the tip of a sharp knife, about 15 minutes. Drain well. Transfer to a rimmed baking sheet, and let stand until cool enough to handle.
Cut off tops and a thin slice from bottoms of 24 potatoes. Using a small melon baller, scoop out flesh from these potatoes, leaving a 1/4-inch-thick shell. Peel remaining 4 potatoes. Press whole potatoes and remaining flesh through a ricer into a medium bowl.
Preheat oven to 375 degrees. Heat milk and the room-temperature butter in a small saucepan over medium until butter melts. Stir into riced potatoes, a little at a time, until creamy. Stir in creme fraiche and the chives; season with salt and pepper.
Transfer to a pastry bag fitted with a 1/2-inch plain round tip (such as Ateco #806). Pipe potato mixture into shells, mounding about 1/2 inch above top edge.
Bake until tops are golden brown and slightly puffed, about 15 minutes. Brush with melted butter, and bake 15 minutes more. Top each potato with about 1/4 teaspoon caviar. Serve immediately.