3 pounds Yukon Gold potatoes, peeled and cut into 2-inch chunks
1 ½ tablespoons kosher salt
½ cup salted butter, cubed
4 large cloves garlic, crushed
1 cup whole milk, warmed
1 cup sour cream
6 tablespoons unsalted butter, cubed
2 teaspoons kosher salt
½ teaspoon freshly ground black pepper
2 tablespoons thinly sliced scallions
Place potatoes and 1 1/2 tablespoons of the salt in a large stock pot. Add enough cold water to cover potatoes by 2 inches. Bring to a boil over high heat. Reduce heat to medium-high and boil until potatoes can be easily pierced with a fork, about 15 minutes.
Meanwhile, place salted butter and garlic in a small saucepan over low heat. Cook, stirring occasionally until garlic is fragrant and white milk solids have separated from butter, 2 to 3 minutes. Remove from heat and cover to keep warm.
Remove potatoes from heat and drain in a colander. Return potatoes to the pot; add milk, sour cream, unsalted butter, 2 teaspoons salt, and pepper. Mash using a potato masher until smooth. (Avoid over-mashing or the starches will form a glue-like texture.) Transfer to a serving bowl.
Pour garlic butter through a fine mesh strainer into a small bowl. Discard milk solids and garlic. Pour butter over mashed potatoes and sprinkle with scallions.