Perfect (And Healthy!) Sour Cream Mashed Potatoes

I'll say it again: Ellie Krieger rocks the show.
I'm loving cooking my way through both of her books right now, as I continue on in my quest to lighten up. Ellie is an upbeat and wonderfully creative nutritionist, and her cooking is spot on, too.
This here is an amalgam of two of her lightened comfort food recipes: Parmesan Mashed Potatoes and Sour Cream Smashed Potatoes. I say: why not both at once?

Ingredients
2 pounds Yukon gold potatoes, diced unpeeled
1/2 cup low-fat sour cream or buttermilk
1/2 cup skim milk
2 tablespoons grated Parmesan
Salt
1 tablespoon unsalted butter

Place potatoes in a large saucepan and cover with water. Bring to a boil, reduce heat and cook until tender, about 10 minutes.

While potatoes are cooking place buttermilk and milk into a small saucepan and cook over a very low heat until just warm. Be careful not to boil or milk will curdle.

Drain the potatoes, add the warmed milk and mash together to desired consistency. Stir in cheese and salt. Top with the butter and let it melt.