- 3 tablespoons butter, plus more for baking dish
- 3 pounds russet potatoes, peeled and chopped
- Coarse salt
- 3 large egg yolks
- 1 1/4 cups milk
- 1 1/2 cups coarsely grated Gruyere, Jarlsberg, or other Swiss cheese (6 ounces)
Preheat oven to 400 degrees. Butter a shallow, 1 1/2-quart baking dish. In a large pot, cover potatoes with water by 1 inch; add 1 tablespoon salt. Bring to a boil; reduce heat. Simmer until easily pierced with a paring knife, about 20 minutes. Drain. Wipe pan; return potatoes.
Add butter; mash with a potato masher until creamy. Cool slightly. Fold in egg yolks, milk, 1 1/4 cups cheese, and 2 teaspoons salt. Spread in prepared dish.
Top with remaining cheese. Bake until golden brown in spots, rotating dish halfway through, about 30 minutes.