Smash 6 peeled garlic cloves with the flat part of a chef’s knife. Place the garlic, 1 cup whole or 2% milk, 1 stick unsalted butter, and 1 teaspoon of the kosher salt in a small saucepan. Cook over low heat until the butter is melted. Remove from the heat and cover to keep warm.
Peel and cut 2 ½ pounds Yukon Gold or russet potatoes into rough 1-inch chunks. Place the potatoes in a medium saucepan and add enough cold water to cover by 1 inch. Add the remaining 1 tablespoon kosher salt and bring to a boil over medium-high heat. Reduce the heat to maintain a simmer and cook until potatoes are very tender and can be easily crushed with a pair of tongs, 15 to 20 minutes. It is better to overcook the potatoes than undercook them.
When the potatoes are ready, drain and return to the pot. Pour the milk mixture through a fine-mesh strainer set over the potatoes, pressing on the cloves to extract as much flavor as possible. Discard the garlic cloves. (Alternatively, use a slotted spoon to remove the garlic cloves.)
Mash with a potato masher until smooth and combined. The potatoes may seem loose at first, but will quickly absorb the milk mixture. Don’t overstir. Top with more butter and finely chopped fresh chives before serving if desired.
