Heat the canola oil in a large pot or Dutch oven over medium heat. Add the onion and a pinch of kosher salt and saute, stirring with a wooden spoon, for about 2 minutes, or until translucent. Add the garlic and bell pepper and saute for 1 minute. Add the chili powder, cumin, and cayenne and cook for at least 1 minute so that they bloom.
Add the tomato paste and cook for 1 or 2 minutes to deepen its flavor.
Add the turkey and switch tools from a wooden spoon to a whisk. Cook the turkey, breaking up any large chunks with the whisk as you cook, for 5 minutes, or until browned.
Add the tomatoes, 3 cups of the stock, and the cocoa powder and bring to a simmer, whisking to make sure the cocoa powder is thoroughly dissolved and combined. Add the beans and bring the chili to a simmer.
Cover the pot with a lid or foil and cook the chili for 1 hour. Taste the chili after 1 hour and adjust the seasoning. If the chili is too thick for your liking, add a bit more chicken stock.
Serve in deep bowls topped with sour cream, sliced scallions, and grated Cheddar, if desired.