Cheesy Baked Mashed Potatoes

Name: Cheesy Baked Mashed Potatoes
Ingredients: Butter or cooking spray, for the baking dish; 3 pounds Yukon gold potatoes (about 8 medium); 3/4 cup whole milk; 4 tablespoons unsalted butter; 1 teaspoon kosher salt, plus more as needed; 1/4 teaspoon freshly ground black pepper; 1 cup sour cream (8 ounces); 8 ounces sharp cheddar cheese, shredded (about 2 cups); 1 ounce Parmesan cheese, finely grated (about 1/2 firmly packed cup grated or 1/3 cup store-bought); Finely chopped fresh chives or parsley, for garnish (optional)