Combine sweet potatoes, butter, 2 tablespoons cream, sugar, and salt in a large pot. Cook, covered, over low heat until potatoes are fall-apart tender (about 35-40 minutes).
Remove from the heat, add remaining 2 tablespoons cream and vanilla extract; mash the potatoes with a potato masher or, for a fluffier, smoother texture, transfer potatoes to a large bowl and beat on high speed with an electric mixer.
Transfer mashed potatoes to a 2-quart baking dish that has been sprayed with cooking spray.
Sprinkle pecans over the potatoes and add marshmallows on top. Bake, uncovered, in a 375°F oven for 15-20 minutes (or until marshmallows are golden brown).
Prefer your Southern sweet potato casserole with no nuts? Just omit the pecans and use only marshmallows on top.
To serve a larger crowd, double all of the ingredients and bake the casserole in a 9 x 13-inch baking dish. The rest of the cooking instructions remain the same.
If you would like some warm spices in your casserole, you can add ½ teaspoon ground cinnamon, ¼ teaspoon ground ginger, and 1/8 teaspoon ground nutmeg to the sweet potatoes at the same time that you add the vanilla extract.
Sweet Potato Casserole with Canned Yams: if you prefer to make your filling with canned yams, see this recipe for those instructions.
Bring the cream to room temperature before adding it to the potatoes. This will prevent the cream from separating when heated.
Slice the potatoes in even widths. This will help them cook evenly so that you don't have some potatoes that are still firm while others are soft.
While sweet potato casserole istypically served as a side dish on a holiday table, adding marshmallows on top makes it feel decadent enough to pass as an easy dessert! For a truly sweet treat, try serving the warm casserole with a scoop of vanilla ice cream.