In a large mixing bowl, whisk together the olive oil, lime juice, minced garlic, cumin, chili powder, paprika, salt, black pepper, and cayenne pepper (if using) to prepare the marinade.
Place the chicken breasts in the bowl with the marinade, ensuring they are fully coated. Cover and refrigerate for at least 15 minutes, or up to 2 hours for enhanced flavor.
Preheat your grill or grill pan over medium-high heat.
Remove the chicken from the marinade and discard the leftover marinade.
Using a meat mallet or rolling pin, pound the chicken breasts to an even thickness for uniform cooking.
Grill the chicken for 6-8 minutes on each side, or until the internal temperature reaches 165°F (75°C).
Once cooked, remove the chicken from the grill and let it rest for 5 minutes.
While the chicken is resting, dice the avocado and set it aside.
Slice the chicken and serve it on a platter. Top with diced avocado and garnish with chopped cilantro.