Drain canned sweet potatoes well and place in a bowl.
Mash sweet potatoes and mix with whipping cream, butter, brown sugar, egg, salt, and cinnamon until smooth.
Spread into a 2-quart baking dish.
Mix together marshmallows, pecans and butter. Sprinkle over the mashed sweet potatoes.
Bake for 20 minutes or until heated through. Cool 5 minutes before serving.
Notes
Make Ahead: Make the casserole. Bake without the topping, cool, and then refrigerate overnight. Sprinkle the topping on and bake until heated through and the marshmallows become toasted.
Leftovers: Refrigerate leftovers in an airtight container for up to 4 days. Reheat covered with foil in a 350°F oven for 15 to 20 minutes or until heated through.
Freezing: Freeze this casserole, covered with plastic wrap, for up to 6 months. Thaw overnight in the refrigerator before reheating.
Alternative Topping: For a different flavor profile, make a streusel topping instead. Mix a cup of flour, ¾ cup brown sugar, and ¼ cup salted butter, and mix until crumbly. Spread on top of the sweet potato mixture and bake.