• 3 pounds Yukon Gold potatoes , scrubbed
• 4 cloves garlic
• 3 teaspoons kosher salt , plus more for egg whites
• 4 tablespoons unsalted butter , softened
• 2 1/4 cups hot milk
• Freshly ground black pepper , to taste
• 5 egg whites

Cut potatoes in half crosswise. Place them in a large pot along with the garlic, and add enough cold water to cover the potatoes by a few inches. Add 1 teaspoon salt and bring water to a boil. Simmer until tender (potatoes should be easily pierced with a paring knife), about 30 minutes. Drain and set aside until cool enough to handle. Slip the skins off and discard.

Preheat the oven to 375°F. Run potatoes and garlic through a ricer or a food mill, into a large bowl. Stir in butter and milk, and season with pepper and 2 teaspoons salt, or more to taste.Preheat the oven to 375°F. Run potatoes and garlic through a ricer or a food mill, into a large bowl. Stir in butter and milk, and season with pepper and 2 teaspoons salt, or more to taste.

In a metal bowl, beat the egg whites with a pinch of salt until soft peaks form. Whisk about a third of the egg whites into the potato mixture, and fold in the rest. Transfer to a 1 1/2- to 2-quart baking dish. (If you're making the potatoes ahead of time, they can be covered and refrigerated at this point.) Bake them about 45 minutes (a little longer if they've been refrigerated), or until the top is golden brown and the potatoes have puffed a bit. Serve immediately.