The potatoes can be cooked and filled up to 2 hours in advance. Transport them on a baking pan so they'll be ready to slide into the oven.
- 12 small Yukon gold potatoes (about 3 pounds)
- 6 large egg yolks
- 4 tablespoons unsalted butter, room temperature
- 1/2 cup heavy cream
- 1 1/2 cups grated Gruyere cheese
- 1 tablespoon plus 2 teaspoons coarse salt
- 1/4 teaspoon freshly ground pepper
Preheat the oven to 425 degrees. Wrap potatoes in aluminum foil, and bake until tender when pierced with the tip of a knife, about 1 hour. Let cool.
When potatoes are cool enough to handle, cut each in half. Scoop out flesh, reserving skins, and pass through a ricer, or mash by hand. Stir in egg yolks, butter, cream, 1 cup Gruyere cheese, salt, and pepper. Scoop filling onto reserved skins. Cover with foil, and refrigerate until shortly before serving.
Preheat the oven to 400 degrees. Place filled potatoes on a baking sheet, and sprinkle with remaining Gruyere. Bake until filling is set and tops are golden brown, about 25 minutes. Serve immediately.