1teaspoonkosher saltplus more for boiling potatoes
4poundsrusset potatoespeeled and cut into 2-inch pieces
½cupwhole milkheated
4tablespoonsunsalted buttermelted with the milk
½teaspoongarlic powder
½teaspoonground black pepper
4ouncescream cheeseroom temperature
¾cup(3 ounces) finely grated Parmesan cheese
2cups(6 ounces) shredded cheddarDIVIDED
Chopped chivesoptional garnish
Instructions
Place potatoes in a large pot, cover with cold water by 2 inches and add a teaspoon or so of kosher salt. Bring to a boil, then reduce to a simmer. Simmer until fork-tender but not falling apart, about 30 minutes; drain.
Just before the potatoes are done, heat milk (on the stove or in the microwave) with the butter, just until the butter is melted and milk is warm.
Transfer potatoes to a large mixing bowl or mixer with the whisk attachment. Mash, then beat (with a hand mixer, or stand mixer) on medium speed until finely crumbled.
Add milk and butter mixture, 1 teaspoon kosher salt, garlic powder, black pepper, cream cheese, Parmesan cheese, and all but ¾ cup of the shredded cheddar.
Mix until the potatoes are smooth and creamy and cheese has just melted. Transfer mashed potatoes to a 2-quart baking dish and top with remaining ¾ cup shredded cheddar.
Place under broiler until cheese topping is just melted, 1 to 2 minutes. Serve, topped with chopped chives if desired.