Preheat oven to 325 degrees F (165 degrees C).

Beat mashed sweet potato, white sugar, eggs, milk, 3 tablespoons melted butter, vanilla extract, and salt together in a large bowl until smooth; spread ito a 9-inch square baking dish.

Bake in preheated oven until firmly set, about 45 minutes.

Mix chopped pecans, brown sugar, flour, and 1/4 cup melted butter together in a bowl until dry ingredients are evenly moistened; spread evenly over the sweet potato mixture and top with marshmallows.

Return dish to oven and bake until marshmallows are lightly browned, about 15 minutes.