Preheat oven to 400°F. Line a baking sheet with foil and place potatoes on it. Prick potatoes all over with a fork and bake for 45 minutes to 1 hour, or until cooked through and soft. Set aside until cool enough to handle.
Peel potatoes, cut into chunks and pass through a ricer. Stir in 6 tablespoons butter, egg yolks and chives; season with salt and pepper.
Line a baking sheet with parchment paper and drop 10 equal sized mounds of potatoes on it, about ½ cup each. Use the back of a spoon to make a small indentation in the center. Place in the freezer until firm, 15-20 minutes. Meanwhile, increase oven to 450°F.
Brush potatoes with egg wash and bake until golden brown on the edges, 20-25 minutes. Brush with remaining 2 tablespoons melted butter, letting some pool in the center swirls. Serve hot.