• Tortilla chips, for serving
Combine the chili powder, coriander, cocoa powder, cinnamon and cayenne in a small dish and reserve.
Heat oil in a large Dutch oven over medium-high heat until shimmering. Add the onion and cook until softened and translucent, 3 to 5 minutes. Add the chopped peppers and cook 3 to 4 minutes. Add the garlic and cook for 1 minute. Push all the vegetables to one side of the pan and sprinkle the cumin seeds into pot. Stirring seeds constantly, toast seeds for 30 seconds, then stir the vegetables and seeds together and cook for 1 minute.
Add the reserved spices to the pot and stir to coat the vegetables, cook until fragrant, about 1 minute.
Add 2 cups water and stir well, scraping bottom of pan to remove any brown spots. Bring to a boil and reduce heat to medium-low and simmer for about 5 minutes, until slightly thickened. Stir in tomatoes, oregano and brown sugar, and bring back to a simmer; cook for 25 to 30 minutes.