In a mixing bowl, combine the cornbread mix, liquid egg, ⅓ cup of milk, diced jalapeño (to taste - if averse to heat, only add half of the jalapeño), and ½ tsp. each of salt and pepper. Using a spatula, mix until no lumps remain and batter is smooth. Transfer batter to prepared casserole dish and place dish on middle rack. Bake for 15-20 minutes, or until cornbread has risen, is golden brown, and a fork comes out of the center cleanly. Remove from oven and allow to cool for 5 minutes before cutting into squares or wedges.