Line a baking sheet with aluminum foil and place it on the lower rack of your oven (to catch any leaks from the potatoes while baking). Stab each sweet potato with a fork 2-3 times to make some holes. Place the potatoes directly onto the oven rack in the center of the oven (the rack with the baking sheet should be underneath). Set the oven to 400 and allow to come to temperature with the potatoes in the oven. Once the oven is heated to 400, continue to bake the potatoes for an hour.
About 20-30 minutes before the potatoes are done, bring a large saute pan to medium heat and add the ground beef. Use a wooden spoon to break it up into small pieces as it cooks. When the meat is breaking up and almost browned, add the onions and the garlic and stir together. Continue to cook another few minutes until the meat is browned and onions are softened.
Add the kidney beans, diced tomatoes, chili powder, salt, cumin and cayenne to the meat mixture and stir together until well combined. Reduce the heat to low and simmer, uncovered, for 10-15 minutes until thickened.
When the sweet potatoes are done, remove from the oven. To serve, slice open a sweet potato and fill with ½ cup of chili. Top with a tablespoon of shredded cheddar and serve.