Preheat the oven to 350 degrees F (175 degrees C).
Place sweet potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and mash.
Mix mashed sweet potato, white sugar, sweetened condensed milk, evaporated milk, melted butter, eggs, vanilla extract, and salt in a bowl using an electric mixer until smooth. Spread into the bottom of a 9x13-inch baking pan.
Mix pecans, brown sugar, melted butter, flour, and sweetened condensed milk in a bowl; spoon topping over sweet potato mixture.
Bake in the preheated oven until golden brown, about 25 minutes.