Add olive oil to a medium saucepan over medium heat. When hot, add onion and bell pepper and cook, stirring, until softened, about 2 minutes. Add garlic and cook an additional 30 seconds, until fragrant.
Add cocoa powder and stir until vegetables are coated. Remove pan from heat. Stir in cubed chicken and enchilada sauce; set aside.
Bring 8 quarts of water to boiling in a large pot. Add spaghetti and cook as directed. Drain pasta and return to warm pot over low heat. Quickly pour in chicken mixture and toss to combine. Add olives, sour cream, and shredded cheese to the pot, stirring frequently until ingredients are evenly distributed and cheese is melted.
Remove pot from heat. Serve hot, with a dollop of sour cream and sprinkle of green onion and cheddar cheese, if desired.