Steam the sweet potatoes, peel them, and mash them.
In a bowl, combine the mashed sweet potatoes, cake flour, baking powder, salt, and sugar.
Add the cold butter cut into small pieces and mix by cutting it in with a spatula.
Gradually add milk to form a dough.
Shape the dough into a thick disc on a floured work surface.
Cut into desired shapes and form the scones.
Place on a baking sheet, brush with egg wash, and bake in a preheated oven at 180°C (356°F) for 20 minutes.