Combine 1/2 tsp cinnamon, 5 tbsp butter, 4 tbsp brown sugar, and flour. Combine until it becomes a sticky, sand like consistency. Set aside.
Make your mashed sweet potatoes. You can do this in the Instant Pot (see recipe coming to the blog in just 2 days), or you can peel and boil them on the stove.
Once boiled and soft, drain them and add them back to the pot with 1 tbsp brown sugar and the remaining butter and cinnamon. Mash until smooth.
Spread your mashed sweet potatoes smoothly and evenly into a large casserole dish.
Sprinkle half of the marshmallows across the top (there will be gaps at this point).
Sprinkle half of the streusel topping across the top.
Top with remaining marshmallows and streusel.
Bake in the oven at 400°F for 6 minutes. WATCH CLOSELY. You do not want the marshmallows to burn.