Rinse the lentils well under cool running water. Remove any debris.
To a 4-quart or larger slow cooker add the lentils, drained beans, diced onion, diced pepper, garlic, cocoa powder, cumin, chipotle chli powder, chili powder, and salt. Stir to combine.
Pour in the tomatoes with their juices, tomato sauce, and vegetable broth. Stir again.
Put the slow cooker lid in place. Adjust heat to "low" and cook for 8 hours. Test a few lentils to ensure that the chili is fully cooked. If not, increase the heat to high and continue slow cooking until ready.
Serve topped with cheddar cheese, cilantro, and sour cream.
Video
Notes
Low vs high heat: I prefer a slow simmer for this chili because I find that it cooks dry lentils more evenly, but it can be made quicker by selecting high heat and cooking for 5 to 6 hours.To make on the stovetop: I recommend sauteing the onion and garlic over medium heat for about 6 minutes, then add in the remaining ingredients. Bring to a rapid simmer over medium-high heat, then reduce to a gentle simmer over medium-low. Cover and simmer until the lentils are tender, about 25 to 30 minutes. Continue simmering to develop flavors, as desired.