Boil, steam or microwave sweet potato until cooked through. Mash into smooth paste adding the soy milk.
In a large bowl, add all the ingredients and knead into soft dough using warm water as needed. Cover with plastic wrap and rest for at least 2 hours. Dough almost doubles in size.
After 2 hours, knead the dough lightly and make large lemon size balls.
Preheat a griddle/ skillet. Take one ball at a time and roll into either an oval or circle shaped discs. Sprinkle water on one side of the disc and place it wet side down on the hot griddle. Cook until bubbles form on the other side of the naan. Flip and cook on the other side until blisters are formed.
Brush the butter on both sides (generously) before serving.
Serve hot with a gravy curry of your choice.