Preheat the oven to 350F.
Peel and dice the sweet potatoes into 1 inch chunks
Cover potatoes with salted water and bring to low boil; cook until soft and tender enough for a fork to pierce easily.
Drain the potatoes, then add butter (I like to break into about 1 tbsp. pieces) and mash well.
Next add brown sugar, cinnamon or pumpkin pie spice and salt to the mashed sweet potato mixture. Stir well incorporating all ingredients. Smooth the top.
In a separate bowl, combine the oat streusel topping. Mix well, (I find it easiest to use hands.)
Crumble the topping across the smoothed out sweet potato mixture.
Bake for 15 minutes then remove and add mini marshmallows, increasing temperature to 425F, cooking until marshmallows melt or are golden brown, whatever your preference—about 5-10 minutes.
