Heat olive oil in a large pot or Dutch oven over medium heat.
Add onion and cook until softened, about 5-7 minutes.
Stir in garlic and jalapeño and cook for 1 minute more until fragrant.
Add chili powder, cumin, and cayenne pepper (if using) and cook for 30 seconds, stirring constantly.
Pour in vegetable broth, black beans, and diced tomatoes.
Bring the soup to a boil, then reduce heat and simmer for at least 20 minutes to allow flavors to meld. For a thicker soup, mash some of the beans against the side of the pot.
Season with salt and pepper to taste.
Serve hot with your favorite toppings.
Notes
For extra spice, leave some seeds in the jalapeño or add a pinch more cayenne pepper.
If you prefer a smoother soup, you can blend a portion of it using an immersion blender or a regular blender (be careful when blending hot liquids).