Place potatoes and 1 tablespoon of the salt in a Dutch oven with water to cover by 1 inch. Bring to a boil over high, and cook until potatoes can be pierced easily with a knife, about 15 minutes. Remove from heat. Drain well, and return potatoes to Dutch oven. Mash potatoes with a potato ricer or potato masher until very smooth. Stir in egg yolks, 1 at a time. Stir in cream, butter, both cheeses, white pepper, and remaining 2 teaspoons salt until smooth. (Cheese may not melt completely.) Transfer mixture to a lightly greased 13- x 9-inch broiler-proof baking dish, and let cool to room temperature, about 1 hour. While potatoes are still soft, smooth top with an offset spatula or create a decorative pattern with a spoon, if desired. Cover with aluminum foil, and chill until ready to bake, up to 1 day ahead.

Preheat oven to 350°F. Remove casserole from refrigerator while oven preheats. Bake, covered, until heated through, about 45 minutes. Uncover and bake 15 minutes. Increase oven temperature to broil on HIGH, and broil until topping is browned, 3 to 4 minutes.