Preheat your oven to 180°C (350°F) and set a large pot of salted water to boil.
Add the peeled and chopped sweet potatoes into the boiling water and cook until very tender, about 15 minutes. You should be able to easily pierce them with a fork.
Drain the cooked sweet potatoes well and transfer them to a large mixing bowl.
Using a masher or fork, mash the sweet potatoes until smooth and creamy, then stir in warm milk and melted butter until well combined and silky.
Mix in brown sugar, cinnamon, and vanilla extract to add warmth and depth of flavor. Taste and adjust sweetness if desired.
Spread the sweet potato mash evenly into a buttered 9x13 inch baking dish, smoothing the top with a spatula.
Sprinkle the chopped pecans evenly over the surface, then top with mini marshmallows for a gooey, crispy topping.
Bake the casserole in the preheated oven for 20-25 minutes, until the marshmallows are golden and bubbling, and the pecans are toasted.
Once baked, remove from oven and optionally sprinkle with a pinch of sea salt or a splash of bourbon for an extra flavor boost.
Allow to cool slightly before serving to enjoy the warm, creamy interior topped with crispy, caramelized marshmallows and crunchy pecans.
