In a small bowl, whisk together 2 tablespoons olive oil, 2 tablespoons lime juice, lime zest, 1 tsp chili powder, 1 tsp brown sugar, 3/4 tsp salt, 1/2 tsp each ground cumin, smoked paprika, onion powder and garlic powder, 1/4–1/2 tsp chipotle chili powder (optional), and 1/4 tsp black pepper to make the chicken rub.
Rub the mixture evenly over both sides of the pounded chicken breasts; if time allows refrigerate up to 8 hours or let sit 30 minutes at room temperature for more flavor.
While the chicken rests, combine the salsa ingredients except the avocados in a large bowl: 1 cup quartered cherry tomatoes (or 2 chopped Roma), kernels from 1 ear fresh corn, 1/3 cup finely chopped red onion, 1/2 chopped red bell pepper, diced jalapeño (seeds separated), 2 tablespoons chopped cilantro, 1 minced garlic clove (or 1/4 tsp garlic powder), 2 tablespoons lime juice, 1/4 tsp ground cumin, 1/4 tsp salt and 1/8 tsp black pepper. Toss to combine and chill, covered, for 30 minutes to meld flavors.
Heat and lightly grease a grill pan or skillet over medium-high heat (or preheat an outdoor grill to medium, about 375–450°F). When very hot, add the chicken and cook undisturbed 3–5 minutes until nicely browned.
Flip the chicken, reduce heat to medium, cover if using a pan, and cook 5–7 more minutes until an instant-read thermometer inserted into the thickest part reads 165°F (cooked through). Remove to a cutting board and let rest 5 minutes before slicing.
Just before serving, gently fold the chopped avocados into the chilled salsa. Taste and adjust salt, pepper, and jalapeño seeds for desired heat.
Slice the rested chicken and serve topped with the avocado salsa and extra lime juice if desired.