Preheat the oven to 350°. Spray or brush oil inside casserole, set a side.
Boil the sweet potatoes until fork tender and soft. About 20 minutes
Drain the potatoes and place them in a mixing bowl. Option to mash the potatoes by hand with a hand masher, or you could whip the potatoes with an electric hand mixer or stand mixer.
Add egg, heavy cream, cinnamon, and brown sugar. Whip together until potatoes look fluffy.
Spoon potato mix into pregreased casserole. Option to smooth potatoes flat or allow for peaks and valleys to form.
Place casserole in oven for about 20 minutes or until casserole reaches an internal temperature of 165°. Remove from oven and serve