Preheat oven to 400°. Place 2 tablespoons butter in a 10-in. cast-iron or other ovenproof skillet. Place in oven until butter is melted, 4-5 minutes; carefully swirl to coat evenly. , Meanwhile, in a large bowl, whisk eggs, milk and sweet potatoes until blended. Whisk in flour and salt. Pour into hot skillet. Bake until puffed and sides are golden brown and crisp, 20-25 minutes., In a small saucepan, combine syrup, pecans and remaining 2 tablespoons butter. Cook and stir over medium heat until butter is melted. Remove pancake from oven; serve immediately with syrup.