The addition of delicious sweet potatoes gives a nutrient boost to mashed potatoes. Double it: Cook the potatoes in a Dutch oven rather than a large saucepan.
Place the potatoes and sweet potatoes in a large saucepan and add enough water to cover. Bring to a boil over high heat and cook until very tender when pierced with a fork, 20-25 minutes.
Drain the potatoes, then mash them in the pot to the desired consistency. Place the milk and butter in a small bowl and microwave on high until the butter is mostly melted and the milk is warm, 30-40 seconds. (Alternatively, place in a small saucepan and heat over medium until the milk is warm.)
Stir the milk mixture, sugar, salt, and pepper into the mashed potatoes until combined.