Ubi Jalar Panggang

Bahan-bahan (6 porsi)

• 4 lbs (1.8kg) sweet potatoes
• 6 oz (175g) unsalted butter (room temperature or melted)
• 1 1/2 teaspoons Maldon sea salt flakes (preferred or salt)
• 1/2 teaspoon freshly ground black pepper
• 2 teaspoons rosemary (or oregano, chopped )