Preheat the oven to 425 degrees Fahrenheit.
Place the scrubbed sweet potatoes on a foil lined baking sheet. Bake the potatoes for 1 hour - 1 hour 20 minutes, or until the sweet potatoes are baked through and are pierced easily with a fork.
Let the sweet potatoes cool at room temperature for 5-10 minutes, or until the skin is cool enough to touch.
Being careful not to burn yourself, scoop the pulp out of the sweet potato skin. Place the sweet potato pulp in the bowl of a stand mixer fitted with the whisk attachment (or in a large mixing bowl if using a hand mixer).
Add the butter, salt, cinnamon, and brown sugar to the sweet potatoes. Whip the potatoes on medium-high speed until light and fluffy. If the sweet potatoes are too dry for your liking, whip in 2-4 tablespoons of milk on low speed or until your desired consistency. Serve the sweet potatoes hot with a pat of butter on top if desired.
