Steam sweet potatoes for 15 to 20 minutes, until you can easily mash them. Meanwhile, mix together the milk, eggs, and butter. Add the remaining ingredients except the yogurt and avocado.
Add mashed sweet potatoes, to the mixture and combine. Heat waffle iron and spray with nonstick cooking spray. Pour 1/6 of the batter into iron, and cook for about 5 minutes per waffle.