Combine the potatoes, broth, butter, 1/2 teaspoon salt, and garlic in the basin of a slow cooker. Cook on high for 3-4 hours, or low for 4-6, or until the potatoes are very tender. Stir occasionally throughout the cooking time to make sure all the potatoes touch the broth at some point.
Mash the potatoes using a hand-held potato masher until no lumps remain. Then, mash in additional salt to taste, sour cream, and whole milk, starting with 1/2 cup, and adding more until the potatoes reach your desired consistency.
Optional: Using an electric hand mixer, fluff the potatoes just for about 20-30 seconds to get that whipped potato texture. Do NOT overmix or you risk your potatoes turning gummy.
Sprinkle on chopped chives, cover the potatoes, put the slow cooker to warm, and then serve.
