Add the sweet potatoes and 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 transfer the potatoes to a large bowl. Set aside.
In a small pot over low heat, bring the butter, cream, and milk to a simmer. Once simmering, remove from the heat.
Pour the cream mixture over the potatoes and mash with a hand masher until smooth. Season to taste with pepper and salt.
Garnish with chives, if desired.
Note: Russet potatoes gives these a lighter texture compared your typical sweet potato mash, but you can swap them out with two additional pounds of sweet potatoes if you’d prefer to keep it classic.