Start by peeling and cubing the sweet potatoes. Place them in a large pot, cover with water, and bring to a boil. Cook until the sweet potatoes are tender, about 15-20 minutes.
Once cooked, drain the sweet potatoes and return them to the pot. Add the brown sugar, maple syrup, coconut milk, vanilla extract, cinnamon, nutmeg, and salt. Mash everything together using a potato masher or a fork until smooth and creamy.
Preheat your oven to 350°F (175°C). Transfer the sweet potato mixture to a greased 2-quart baking dish, spreading it evenly.
In a mixing bowl, combine the chopped pecans, brown sugar, flaked coconut, whole wheat pastry flour, oats, cinnamon, nutmeg, and salt. Mix in the softened butter until the mixture resembles coarse crumbs.
Sprinkle the topping mixture evenly over the sweet potato layer. Then, scatter mini marshmallows on top for an extra touch of sweetness.
Place the casserole in the preheated oven and bake for 25-30 minutes, or until the topping is golden brown and the marshmallows are toasted.
