Prep and boil the potatoes: Heat the oven to 350°F (175°C). Peel and cube the sweet potatoes. Place them in a large pot, cover with cold water, add a pinch of salt, and bring to a boil. Simmer until very tender, 15–20 minutes. Drain well.

Mash for a smooth base: Return the potatoes to the warm pot. Let steam off excess moisture for 2 minutes. Add 4 tablespoons butter, brown sugar, maple syrup, 1/2 teaspoon salt, cinnamon, nutmeg, and vanilla. Mash until smooth. Stir in milk or half-and-half, starting with 1/3 cup and adding more if needed for a creamy texture.

Add eggs for structure: Once the mixture is warm, not hot, stir in the beaten eggs. This helps the casserole set without turning dense. If the potatoes are too hot, let them cool a few minutes before adding the eggs.

Spread into a dish: Grease a 9x13-inch baking dish. Spread the sweet potato mixture evenly into the dish, smoothing the top.

Make the pecan crumble: In a bowl, mix flour, granulated sugar, a pinch of salt, and the remaining 4 tablespoons melted butter. Stir in chopped pecans. The mixture should look like coarse crumbs.

Add the topping: Sprinkle the pecan crumble evenly over the sweet potatoes. If using marshmallows, you can either skip the crumble, or use half crumble and add marshmallows in the last 10 minutes so they toast without burning.

Bake: Bake for 25–30 minutes, until the edges are set and the topping is golden. If adding marshmallows at the end, scatter them on top after 20 minutes and return to the oven for 8–10 minutes, until puffed and lightly browned.

Rest and serve: Let the casserole rest for 10 minutes before serving. This helps it slice neatly and keeps the texture creamy.