Boil the sweet potato peel and mash; Sweet potatoes can be cooked in a microwave. Wrap the potatoes in wet paper towel and cook for 7-8 minutes.
Heat butter in heavy bottom pan over medium heat. Add sweet potatoes and stir fry by stirring continuously and pressing down for about 8-10 minutes.
Add milk and mix it well. After milk comes to a boil cook for 2 minutes, add sugar and cook for additional 6-8 minutes or until Halwa should start leaving the side of the pan. Halwa should be very moist.
Turn off the heat and add cardamom, cinnamon, and nutmeg. Mix it well.
Serve hot or at room temperature. Garnish with cashews. Halwa tastes best when served hot.