Place the chicken breasts in a zip-top bag with the taco seasoning, cumin, 1/4 teaspoon salt, juice of half a lime, and 1 tablespoon olive oil; seal and shake to coat. Marinate at room temperature for 10 minutes or refrigerate up to 2 days.
Heat a heavy skillet over medium-high until very hot, then add 1 teaspoon oil. Cook the chicken breasts 4–5 minutes per side until cooked through and browned.
Remove the chicken to a plate and let rest in the pan or on a cutting board for at least 5 minutes before slicing.
Meanwhile, make the dressing by blending 1/2 cup sour cream (or Greek yogurt), 1/4 cup packed cilantro, juice of 1/2 lime, and 1/4 teaspoon salt until the cilantro is fully chopped and the dressing is smooth.
Assemble the salads by dividing 2 cups chopped romaine between plates. Top with sliced avocado, diced tomato, 1/2 cup black beans, 1/2 cup corn, 1/4 cup cilantro, and sliced chicken.
Drizzle or dollop the cold creamy cilantro dressing over each salad just before serving.