Heat oven to 350 degrees. Spray a 9x13 baking dish with cooking spray. Set aside.
Place the peeled and chopped sweet potatoes in a large pot and cover with cold water. Bring to a boil and let cook for about 20 minutes (possibly longer). You want the potatoes to be super soft. Drain potatoes in a colander when done cooking.
Add the sugar, butter pieces, and milk into the bottom of a large mixing bowl. Dump the drained and hot sweet potatoes on top. Use a handheld blender to beat the mixture until creamy and smooth.
Add in eggs, one at a time, beating after each egg.
Add nutmeg, cinnamon, salt, and vanilla extract. Blend together until everything is combined and potatoes are creamy and smooth. Evenly spread the sweet potatoes inside the prepared baking dish.
Make the crumble topping: In a small mixing bowl, stir together the flour and brown sugar. Add butter pieces and mix together with a pastry blender or your hands (hands are easier) until mixture resembles wet, coarse crumbs. Add pecans, coconut, and marshmallows. Stir together to combine. Sprinkle over the sweet potatoes.
Bake for 35-40 minutes. The top should be golden brown and edges bubbly.
Let cool for about 10 minutes before serving. Store leftovers, covered in the fridge, for up to 1 week.