Step 1

Place potatoes in a large pot and pour in water to cover by 2"; season with salt. Bring to a boil over medium-high. Reduce heat and simmer until a paring knife very easily slides through potatoes, 25–35 minutes. Drain and let cool slightly.

Step 2

Preheat oven to 425°. Whisk egg yolks, garlic, cream, sour cream, butter, and peppercorns in a large bowl; season generously with salt. Peel potatoes and pass flesh through a ricer or a food mill (or mash them by hand if you don’t mind a few lumps) directly into bowl with egg mixture. Fold in gently (be careful not to overmix). Transfer to a 3-quart baking dish and decoratively shingle or swirl surface. Bake, rotating once, until golden brown and slightly puffed, 30–40 minutes.

Step 3

Do Ahead: Dish can be assembled 1 day ahead. Cover and chill. Increase bake time by 5–10 minutes.