Preheat the oven to 350 degrees F (175 degrees C).
Place a steamer basket in a large pot and bring 2 inches of water to a boil. Place whole sweet potatoes in the steamer basket and cover. Cook until tender, about 30 minutes. Drain and cool.
Peel and slice sweet potatoes lengthwise into 1/2-inch slices. Place in a 9x13-inch baking dish.
Melt brown sugar, butter, water, and salt in a small saucepan over medium heat. When the sauce is bubbly and sugar is dissolved, pour over the potatoes.
Bake in the preheated oven for 1 hour, occasionally basting the potatoes with the brown sugar sauce.