Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until softened, about 5 minutes. Stir in the minced garlic and cook for 1 minute more.

Add the black beans, diced tomatoes, chili powder, cumin, and salt to the pot. Stir to combine and bring the mixture to a simmer. Cook uncovered for 15 minutes, stirring occasionally, until the chili thickens.

While the chili simmers, preheat the oven to 400°F. In a mixing bowl, whisk together the cornmeal, flour, and baking powder. In a separate bowl, mix the milk, egg, and melted butter. Combine the wet and dry ingredients until just mixed.

Pour the cornbread batter into skull-shaped molds or a greased baking pan. Bake for 18 to 20 minutes until the tops turn golden and a toothpick comes out clean. Let the skulls cool slightly before removing from the molds.

Ladle the chili into bowls. Place the sour cream in a piping bag or zip-top bag with one corner snipped. Pipe concentric circles on the chili surface, then drag a knife through the circles to form a web pattern. Serve with the cornbread skulls on the side.