Step 1: Prepare the Sweet Potatoes

1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or nonstick spray and set aside.
2. Bring a large pot of salted water to a boil. Add the peeled and cubed sweet potatoes and cook for 12-15 minutes, or until fork-tender. Drain well.

Step 2: Mash the Sweet Potatoes

1. Transfer the cooked sweet potatoes to a large mixing bowl. Use a potato masher or electric mixer to mash the potatoes until smooth and creamy.
2. Add the melted butter, brown sugar, milk, eggs, vanilla extract, cinnamon, nutmeg, and salt to the bowl. Mix until well combined and fluffy.

Step 3: Assemble the Casserole Base

1. Spread the mashed sweet potato mixture evenly into the prepared baking dish. Smooth the top with a spatula.

Step 4: Prepare the Streusel Topping

1. In a medium bowl, combine the flour, brown sugar, and chopped pecans.
2. Add the softened butter and mix with a fork or your fingers until the mixture forms coarse crumbs.

Step 5: Add the Toppings

1. Sprinkle the pecan streusel evenly over the sweet potato layer. If using marshmallows, add them on top of the streusel or as a separate layer.

Step 6: Bake the Casserole

1. Bake in the preheated oven for 25-30 minutes, or until the topping is golden brown and the marshmallows (if used) are toasted.
2. Let the casserole cool for 5 minutes before serving.