Preheat oven to 350 degrees F (175 degrees C). Spray an 8x8-inch baking dish with cooking spray.

Heat olive oil in a large skillet over medium heat; cook and stir green bell pepper, onion, and celery in hot oil until onion is translucent, about 5 minutes. Stir in hot dog pieces and cook and stir until hot dogs begin to brown, 6 more minutes. Stir in the chili and heat the mixture thoroughly.

Beat cornbread mix, egg, and milk in a bowl with a spoon until batter is just moistened, and spread half the cornbread batter into the prepared baking dish. Top batter with the hot dog mixture; spread remaining half of cornbread batter over the top.

Bake in the preheated oven until casserole is bubbling and cornbread topping is golden brown, about 25 minutes. Let stand for 5 minutes before serving.

Serve with a topping of sour cream and salsa if desired.