Heat olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add diced onion and bell peppers. Sauté for 5-7 minutes until softened.

Add minced garlic, diced mushrooms, and finely chopped walnuts (if using). Cook for another 5-7 minutes, stirring occasionally, until mushrooms have released their moisture and started to brown slightly.

Stir in chili powder, ground cumin, smoked paprika, dried oregano, cayenne pepper, salt, and black pepper. Cook for 1 minute, stirring constantly, until fragrant.

Pour in crushed tomatoes, diced tomatoes (undrained), and vegetable broth. Stir well to combine.

Add rinsed and drained kidney beans, black beans, and pinto beans to the pot. Stir in the unsweetened cocoa powder. Bring the chili to a gentle simmer, then reduce heat to low, cover, and cook for at least 30 minutes, or up to 1 hour, stirring occasionally, to allow the flavors to meld.

During the last 5-10 minutes of cooking, stir in the frozen corn. Continue to simmer until the corn is heated through and the chili has thickened to your desired consistency.

Taste and adjust seasonings as needed. If the chili is too thick, add a splash more vegetable broth.

Ladle the hearty vegetarian chili into bowls. Serve hot with your favorite toppings such as fresh cilantro, diced avocado, a dollop of vegan sour cream, shredded vegan cheese, or a squeeze of lime juice.