Preheat your oven to 350°F (175°C).
Peel and thinly slice the sweet potatoes.
In a bowl, whisk together the heavy cream, brown sugar, vanilla extract, cinnamon, nutmeg, and salt.
Grease a baking dish with the butter.
Layer the sliced sweet potatoes in the baking dish, pouring some of the cream mixture over each layer.
Finish by pouring any remaining cream mixture over the top and dot with additional butter.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 30-40 minutes, or until the top is golden and bubbly.
Let the scalloped sweet potatoes rest for about 10 minutes before serving.