Wash the sweet potatoes thoroughly, peel them, and cut into suitable sizes.
Place the sweet potatoes in a steamer and steam until soft.
Put the steamed sweet potatoes in a bowl and mash them.
Add mayonnaise, salt, and sugar to the mashed sweet potatoes and mix well.
Generously spread the sweet potato mousse on a slice of bread, then cover with another slice.
Cut into bite-sized pieces to serve.