Bake potatoes in 400 degree oven for about an hour or until tender. Cool. Scrape out the insides and mash until smooth.
Add 3/4 cup brown sugar, heavy cream, melted butter, 1/4 teaspoon cinnamon, and salt into the sweet potatoes and mix well with electric beater. Pour this mixture into square baking dish. Preheat oven to 350 degrees.
Make the oatmeal streusel. Pulse the oats in a food processor or blender. Combine the pulsed outs with 4 tablespoons brown sugar, flour, and 1/4 teaspoon cinnamon in a bowl. Cut the cold butter into the dry mixture using a fork. You should have a crumbly mixture with pea-size bits. Sprinkle this oatmeal streusel over the sweet potato mixture and bake for 70 to 80 minutes or until beginning to brown. When you remove the casserole from the oven, spread the marshmallows over the top. Let stand for ten minutes.