Grease the pan with butter or nonstick cooking spray.
Remove the skin and thinly slice your sweet potatoes.
Combine the egg, cream, sugar, 3 tbsp melted butter and vanilla into a medium sized bowl to make the custard.
Pour the custard over the sweet potatoes, mixing it thoroughly to coat all the potatoes.
Sprinkle cinnamon over top of the potatoes and custard mixture.
Combine the crumble topping ingredients in a small bowl and sprinkle over the top of the covered potatoes.
Cover the dish with foil and bake for 55 mins.
Remove the casserole from oven and turn on broil or to the highest setting.
Cover the top with marshmallows and put back in the oven for 2-3 mins to allow the marshmallows to get nice and toasted.
Notes
Double the Recipe: This is a great recipe for a holiday or family gathering. Simply double the ingredients and split it into two different baking dishes before baking for the best results.
Softer Casserole: If you want a soft, smooth casserole try using canned sweet potatoes instead of fresh ones.
Add bourbon for flavor: If you want to add a fun twist on your casserole try adding in some bourbon. Just add about half a cup to the custard mixture for a different flavor.
Different nuts: If pecans aren't your preferred nut of choice, then you can easily switch out and add in a different nut. Walnuts would be a great option for this recipe.
Mashed: It is easy to mash the potatoes with the potato masher for a smoother consistency to the mashed sweet potatoes.