Prepare the Cornbread Base:
Preheat oven to 375°F (190°C).
Grease a 9x13-inch baking dish.
In a bowl, mix together Jiffy cornbread mix, egg, milk, cream-style corn, and shredded cheddar cheese until combined.
Pour the mixture into the prepared baking dish and bake for 15 minutes, until partially set (it will continue baking later).
Prepare the Filling:
While cornbread bakes, heat a large skillet over medium-high heat.
Add ground beef and diced onion, cooking until browned (5-7 minutes). Drain excess fat.
Stir in garlic, taco seasoning, cumin, chili powder, Rotel tomatoes, black beans, and corn. Cook for 2-3 minutes, then remove from heat.
Assemble the Casserole:
Remove the partially baked cornbread from the oven.
Spread the beef mixture evenly over the cornbread base.
Sprinkle with shredded Mexican cheese blend.
Bake And Serve:
Return to oven and bake for 15-20 minutes, until the cheese is melted and bubbly.
Let cool for 5 minutes, then garnish with chopped cilantro.
Serve warm with sour cream and sliced jalapeños on top!