Boil sweet potatoes in salted water until tender, about 20 minutes.

Heat oven to 350°F. Grease a 9" casserole dish with butter.

Drain the sweet potatoes and allow them to cool, or run them under cold water in the colander.

In a large bowl, combine sweet potatoes, egg, cream, butter, salt, and pumpkin pie spice. Use a potato masher or large fork to mash the sweet potatoes.

Beat on medium speed until the mixture is smooth and creamy.

Spread the sweet potato mixture evenly in the prepared casserole dish.

In a medium bowl, combine the pecans, brown sugar, and butter until crumbly.

Spread the pecan mixture evenly over the sweet potato mixture in the casserole dish.

Bake for 30 minutes. Allow the casserole to cool for 10 minutes before serving.