1. 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.

2. 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.

3. 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.

4. 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.

5. 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.