Preheat the oven to 400 degrees.
Grease a 9x9" or an 8x8" baking dish; set aside.
Pour the coconut milk into a large bowl. If your coconut milk has a lot of solids at the top of the can, pour the milk into a microwave-safe bowl and heat in the microwave for 30 seconds to soften.
Add the cumin, salt, cardamom, turmeric, coriander, ginger, chili powder (if using), garlic and onion powders. Whisk all the ingredients together and taste for seasoning. Adjust as desired.
Peel the sweet potatoes and slice them on a mandoline using the "thin" attachment. Add them to the coconut milk-spice mixture. Using your hands or a spoon, combine everything so all the slices are coated. Disposable gloves are helpful for this.
Using a silicone spatula, scrape all that wonderfully-spiced mixture into your baking dish.
Cover the dish with foil and bake for 30 to 35 minutes for a 9x9" dish, or 40-45 minutes for an 8x8" dish. (Be aware that baking times are approximate. Length of baking time also depends on the thickness of your sweet potato slices. Adjust as needed.)
Remove the foil and continue to bake for another 15 minutes.
Once bubbly and the sweet potatoes are easily pierced with a fork, remove the dish from the oven and allow it cool slightly.
Sprinkle with the cilantro and serve very warm. Serve with naan bread, to soak up that awesome sauce.