Add the potatoes to a large pot and cover with cold water. Add the salt. Bring to a boil over high heat and cook until the potatoes are fork-tender, about 20 minutes longer. Drain and return to the pot. Set aside.
In a small pot over medium heat, bring the butter, cream, and garlic to a simmer. Once simmering, remove from the heat.
Pour the cream mixture over the potatoes. Add the sour cream, then mash with a hand masher until the potatoes are creamy, but still very chunky. Season to taste with salt and pepper.
Garnish with green onions, if desired.
Note: Any small waxy potato would work well in this recipe — such as Yukon Golds or new potatoes.