Garlic Parmesan Mashed Sweet Potatoes

Bahan-bahan (8 porsi)

• 4 pounds sweet potatoes, peeled and cut into 1-inch pieces
• fine sea salt and freshly-ground black pepper
• 1/4 cup butter
• 4 cloves garlic, pressed or minced
• 3/4 cup milk, warmed
• 1/3 cup sour cream
• 1 ounce freshly-grated Parmesan cheese, plus extra for garnish
• 2 tablespoons chopped fresh chives