Place the potatoes in a Dutch oven with cold water to cover and 1 tablespoon salt. Bring the water to a boil over medium-high heat, turn the heat to medium-low, and simmer, uncovered, 10 to 15 minutes or until the potatoes fall apart easily when pierced with a fork.
Meanwhile, in a small saucepan heat the buttermilk, milk, butter, and garlic over low heat. Make sure it doesn't boil. Keep the liquid warm until the potatoes are done.
Drain the potatoes in a colander, then transfer them to a large bowl. Using an electric mixer, whip the potates while slowly pouring in the hot buttermilk mixture. Add enough of the mixture to make the potatoes creamy. You may need more or less buttermilk depending on how dry or wet the potatoes are. Mix in the chives, 1 teaspoon salt, and pepper. Season to taste. Garnish with additional chives, if desired.