Wash, peel and cut potatoes in cubes. Place them in a large pot. Cover potatoes with water.
Bring to the boil. Cook 15 minutes on medium heat.
Drain potatoes in sieve or colander. Transfer them to the large bowl.
Using the fork or hand mixer, mash or blend until desired consistency.
Add butter and continue mashing until incorporated.
Add sugar, spices and salt. Mix everything.
Crush half of pecans with hands or knife. Mix in the sweet potato mash.
Transfer mashed potatoes to the 7 x 10 inch / 18 x 25 cm baking dish. Gently press and smooth the top.
Crush leftover pecans and sprinkle on top of casserole.
Top with mini marshmallows.
Bake 20 minutes until marshmallows are golden brown and filling is bubbly. Let it cool 10 minutes before serving.
You can freeze the sweet potato casserole without marshmallows up to two months.Store leftover casserole up to 4 days in the fridge. Reheat it about 10 minutes in the oven (355 F / 180 C10 or microwave for about 2-3 minutes.