Sweet Potato Puree with Caramelized Marshmallow

INGREDIENTS

• 6 lb. sweet potatoes
• 16 Tbs. (2 sticks) unsalted butter, plus more for baking dish
• 1 cup heavy cream
• 1/4 tsp. freshly grated nutmeg
• Kosher salt and freshly ground pepper, to taste
• 1 cup water
• 1 1/2 Tbs. gelatin powder
• 1 cup plus 2 Tbs. sugar
• 1 Tbs. light corn syrup
• 2 egg whites