Peel and chop the sweet potatoes into small pieces.
Add to a large pot, and cover with about an inch of cold water. Bring to a boil, and simmer for 15-20 minutes until very soft.
Drain the and return sweet potatoes to the pot. Mash lightly.
Add the butter, milk, pumpkin pie spice, brown sugar and salt to the potatoes.
Continue mashing until smooth.
Transfer the sweet potatoes to an 8x8 casserole dish and keep warm if serving right away.
Topping
If you bought whole pecans, chop them into small pieces.
In a small bowl, combine butter, brown sugar, flour, pumpkin pie spice, salt and pecans until thoroughly combined. See pic for texture.
Assemble/Bake
Top with half the marshmallows, then sprinkle the streusel over the marshmallows. Top with the other half of the marshmallows. You should be able to see both marshmallow and streusel.
Bake at 375 for 7-10 minutes, until the topping is bubbly and brown (keep an eye on it so the marshmallows don't burn).
Devour.
Notes
While you make the topping, preheat oven to 375°F.Heat your liquid (half and half, cream, milk). Hot milk is better than cold, as it will keep your potatoes piping hot!Boiling sweet potatoes will take less time if you cut them into smaller chunks.How to tell if my sweet potatoes are cooked properly?You'll know they are cooked if you can mash one against the side of the pot. You can use a knife or fork, too. They need to be soft all the way through (or you'll have lumps!)What do do with leftoversStore in a covered container in the refrigerator for up to 4 days. Reheat until piping hot!Substitutions/Additions
You can use milk, sour cream or greek yogurt instead of half and half. Heavy cream can also be used.
You can sub cinnamon for pumpkin pie spice if you don't have it. But if you want to make your own pumpkin pie spice, I got you!
Making aheadIf you are making these for a holiday - you can make them ahead. Simply pour the mashed sweet potatoes into an 8x8 baking dish, and cover. Refrigerate. Heat the casserole in the oven until the sweet potatoes are warm, then add the topping and bake for 10-15 minutes, until the topping is bubbly and brown (keep an eye on it so the marshmallows don't burn).A crock pot/slow cooker will also keep these warm if you make them earlier in the day, but the topping won't brown (just melt).