Preheat the oven to 400 degrees. Spray a 9x13 baking dish with nonstick cooking spray. Set aside.
Clean the sweet potatoes to remove any dirt on the outside. Pierce with the fork about 4 times on each side. Wrap in foil. Place the sweet potatoes in the oven and bake for about 45 minutes or until soft. Remove the sweet potatoes and reduce the oven temperature to 350 degrees.
Carefully remove the foil and then remove the skins from the sweet potatoes. Add the sweet potato pulp to a large bowl and mash lightly. Stir in 3/4 cup brown sugar, 1/4 cup butter, 2 tsp salt and 1/2 tsp vanilla. Mix well.
In a small bowl, combine 3/4 cup flour, 1/2 cup brown sugar, 1 tsp salt and chopped pepitas to bowl. Using your hands or a pastry tool, blend in 8 Tbsp of butter.
Combine marshmallows and cinnamon in a plastic bag. Seal bag and shake to coat the marshmallows in cinnamon.
Cover the sweet potato mixture with marshmallows and streusel. Bake in 350 degree oven for 20 minutes.