Mix water, sugar, and active yeast and let stand for 10 minutes or until foamy. If it doesn’t foam, your yeast is dead and you’ll need to throw that out and start over. Whisk in buttermilk, melted butter, and salt. Whisk in flour until smooth. Wrap bowl tightly with plastic wrap and leave on the counter overnight. Be sure to allow lots of room for expansion in the bowl. The batter will at least double in size. I learned that lesson the hard way and came into my kitchen the next morning to find waffle batter dripping from my kitchen counter. In the morning, add eggs, vanilla extract, baking soda, sweet potato, and ground cinnamon to a small bowl. Whisk until smooth. Add sweet potato mixture to flour mixture and whisk just until well incorporated. Prepare waffles according to manufacturer’s recommendations. Serve warm. For batches, keep waffles warm by placing them on a baking sheet in a 200°F oven. They’re best freshly made and eaten straight from the waffle maker. If folks are lined up at your waffle maker and complaining they might perish waiting on their turn, give them a piece of fruit to tide them over. Only use the batch method in case of emergencies. I’m not sure what constitutes a waffle emergency. That's for you to decide.