Preheat oven to 375°F (190°C) and grease a 9x9-inch or similar baking dish.
In a bowl, whisk the egg, milk and oil until combined.
Add cornbread mix and stir just until moistened; do not overmix.
Fold in sour cream, salsa, cumin and chili powder until blended.
Stir in drained corn and rinsed black beans, reserving some cheese for the top.
Mix in half the shredded cheese, green onions and chopped cilantro; season with salt and pepper.
Pour the batter into the prepared dish and smooth the top with a spatula.
Sprinkle remaining cheese evenly over the batter.
Bake 25–35 minutes until set and golden on top; a toothpick inserted should come out mostly clean.
Let rest 5–10 minutes, then top with sliced jalapeños and extra cilantro before slicing and serving.