Scrub sweet potatoes and either cook them in the microwave or boil them. Microwave: Place on a microwave-safe plate and cook on full power for about 7-10 minutes (depending on the microwave potency) or until they turn out soft.
Then, peel them by gently scrubbing the peel with a pairing knife. Or alternatively, cut them lengthwise and scoop all the flesh out of the peel using a spoon or cookie scoop. Just be careful to avoid burning your hands!
To boil: Peel them first, cut into chunks, and place into a pot filled with cold water (never boiling water; otherwise they won't cook evenly). Bring to a boil and let cook for about 12-15 minutes or until soft. Drain!
Meanwhile, preheat oven to 350º F. Mash sweet potatoes and stir in brown sugar, cinnamon, vanilla, melted butter, milk, and egg yolks until obtaining a homogeneous mixture.
Pour into a 2-quart casserole dish coated with cooking spray. Bake on middle rack for about 35 minutes. Five minutes before removing casserole from the oven, prepare the meringue.
Beat egg whites with cream of tartar in the bowl of a mixer until foamy, about 1-2 minutes.
Add powdered sugar and beat again until stiff, about 1-2 minutes. Increase heat to 400 F.
Spoon meringue on top of baked casserole and bake on top rack of the oven for additional 6-7 minutes or until meringue is golden brown. Serve and enjoy!