For the chili: Heat the oil in a 12-inch cast-iron skillet over medium heat. Add the garlic and onion and cook until softened, 2 to 3 minutes. Add the butternut squash and peppers and cook for about 5 minutes more. Stir in the chili powder, salt, oregano, cumin and black pepper to taste. Cook until the spices are aromatic, 2 to 3 minutes more. Pour in the salsa, black and pinto beans and tomatoes, scraping up any browned bits on the bottom of the skillet. Add the vegetable stock and bring to a simmer. Reduce the heat to low and simmer until the vegetables are tender, 30 to 45 minutes.
Preheat the oven to 425 degrees F.
For the cornbread: Meanwhile, in a large bowl, whisk to combine the cornmeal, flour, sugar, baking powder and salt. Make a well in the center of the dry mixture and add the melted butter, egg and buttermilk. Whisk to combine, taking care not to overmix. Use a rubber spatula to scrape the sides of the bowl to ensure that all the ingredients are incorporated.
Use a tablespoon or ice cream scoop to dollop the cornbread mixture over the top of the chili in the skillet. Place the skillet in the oven and bake until the cornbread is golden brown on top, 25 to 30 minutes.