MAKE THE SWEET POTATO MASH
Add peeled and cubed sweet potatoes to a pot with salted water an inch above the potatoes.
Bring the water to a boil and cook for 15 to 20 minutes or until the potatoes are tender with a fork.
Turn off the heat, drain all the water, and add the potatoes back to the pot so the residual heat can dry any excess water.
Mash the sweet potatoes until there are no more chunks.
To a pan over medium heat, add the butter. Cook for 5 to 6 minutes until it starts to bubble and then brown. Make sure not to cook it any further or it can burn. Transfer it to a bowl and allow it to cool so it's not hot.
Add the brown butter to the sweet potato mash along with the brown sugar, maple syrup, heavy cream, salt, vanilla extract, nutmeg, and cinnamon. Mix together until smooth and all incorporated evenly.
MAKE THE STREUSEL
Add brown sugar, all purpose flour, cubed butter, cinnamon, and salt. Mix together using a fork, pastry blender, or your hands until the consistency is like coarse crumbs.
ASSEMBLE AND BAKE
Preheat oven to 350°F
Transfer the sweet potato filling to a casserole dish. Crumble the streusel topping on top along with the chopped pecans.
Bake in the oven for 2o to 30 minutes or until the crumble is golden and crispy.
Take out of the oven, add the marshmallows on top, and then pop back in the oven to roast for another 10 minutes or until the marshmallows are golden on top.
Remove from the oven and allow to cool for 5 to 10 minutes before serving warm!