Creamy Dreamy Potato Casserole: My Family's Fav - Chef YoYo

Ingredients

• 3 pounds Yukon Gold potatoes, peeled and cubed (approximately 1.36 kg)
• 1 teaspoon salt (plus more for seasoning water)
• ½ teaspoon black pepper
• 4 tablespoons unsalted butter (56g)
• ¼ cup all-purpose flour (30g)
• 3 cups whole milk (710 ml)
• 8 ounces cream cheese, softened (227g)
• 1 cup sour cream (227g)
• 1 teaspoon garlic powder
• ½ teaspoon onion powder
• ¼ teaspoon ground nutmeg (optional, but adds a lovely warmth)
• 1 cup shredded cheddar cheese, divided (113g)
• 4 tablespoons unsalted butter, melted (56g)
• 1 ½ cups crushed cornflakes (or Ritz crackers for a more savory option, 100g approx.)
• ¼ cup grated Parmesan cheese (25g)
• 2 tablespoons chopped fresh chives (optional, for garnish)