Preheat the oven to 350°F (175°C) and grease a 9x13-inch ceramic baking dish.

In a large stockpot, cover cubed sweet potatoes with cold water. Bring to a rolling boil, then reduce heat and simmer for 15 minutes or until fork-tender.

Drain the potatoes thoroughly in a colander and return them to the warm pot to allow residual moisture to evaporate for 1 minute.

Mash the potatoes using an electric hand mixer on medium speed until no large lumps remain.

Add melted butter, milk, brown sugar, vanilla extract, cinnamon, nutmeg, and salt. Whisk until the mixture is uniform and creamy.

Incorporate the lightly beaten eggs, stirring rapidly to ensure they are fully integrated without scrambling from residual heat.

Transfer the mixture to the prepared baking dish and smooth the surface with a spatula. (Note: For make-ahead preparation, cover and refrigerate at this stage for up to 24 hours).

Bake for 25-30 minutes until the casserole is heated through and the edges begin to pull away slightly from the sides.

Distribute the chopped pecans and miniature marshmallows evenly across the top of the potatoes.

Return to the oven for 5-8 minutes, or until the marshmallows are puffed and achieve a golden-brown toasted color. Monitor closely to prevent burning. Serve immediately.