Place potatoes in a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, 20 to 25 minutes. Drain.
Mash potatoes with a potato masher or an electric mixer on low in a bowl until smooth.
Heat butter and half-and-half in a small saucepan over medium heat until butter is melted and half-and-half is warmed, 3 to 5 minutes.
Mash or beat butter mixture, salt, black pepper, and garlic powder into mashed potatoes until potatoes are light and fluffy. Stir additional half-and-half, 1 tablespoon at a time, into potatoes for creamier consistency.