Rinse sweet potatoes with tap water.

Steam sweet potatoes whole. Add water to a food steamer and place the steaming rack inside. The water level should be below the steaming rack. Place sweet potatoes on the rack without overlapping them. Cover and bring to a boil over high heat. Turn to low heat. Cook until the sweet potatoes can be easily pierced with a chopstick, about 30 minutes (the time will vary depending on the size of the potatoes).

(Optional) If you don’t have a steamer, place a bowl (or deep plate) upside down in a large pot or skillet. Place a plate on top of the bowl. Add water until it covers the bowl. Place a few chopsticks on the plate to form a rack. Place sweet potatoes on the rack. Cook in the same way.

Preheat oven to 200 degrees C (400 F).

When the sweet potatoes are cooked through, transfer to a plate to cool. When the sweet potatoes are cool enough to handle, but still quite hot, carefully peel the skin by hand and discard it.

Transfer to a large bowl and chop into several pieces. Add butter, milk, and salt. Mash and mix until fully incorporated and the texture is consistent.

Transfer the sweet potatoes to a 1 liter (1 quart) baking dish. Spread cheese on top.

Bake at 200 degrees C (400 F) for 10 minutes, until the cheese turns golden. Transfer to kitchen counter to cool off for 5 minutes.

Serve warm.