Mix the burgers: In a large mixing bowl or the bowl of your electric mixer, combine the cooled sweet potatoes and millet, black beans, onion, cilantro, cumin, chili powder, chipotle or paprika, cayenne (optional, add to taste for spicier burgers) and salt. Use a potato masher, big mixing spoon or the paddle attachment of your mixer to mix really well. It's ok if the black beans get smashed in the process.
Grind the oats: Use a food processor or blender to grind the oats until the flakes are broken up, but not as fine as flour.
Cook the millet: In a small saucepan, bring 1 cup of water to boil. Stir in the millet, reduce heat to low and simmer, covered, until tender (about 25 minutes). Drain off any remaining liquid and set aside to cool. OR cook the quinoa: Rinse the quinoa in a fine mesh colander, then combine with ⅔ cup water in a small saucepan. Bring the mixture to boil, then cover and reduce heat to maintain a gentle simmer. Simmer for 15 minutes, then remove from heat and let the quinoa steam with the lid on for 5 minutes. Then drain off any excess water and set aside to cool.
Roast the sweet potatoes: Preheat the oven to 400 degrees Fahrenheit. Slice the sweet potatoes down the center lengthwise. Place the sweet potatoes cut side down on a rimmed baking sheet. Roast until they yield to a gentle squeeze, 30 to 40 minutes or longer. Once the sweet potatoes are cool enough to handle, remove the skin (it should pull off easily) and roughly chop the insides. Set aside to cool completely.