Whisk together marinade ingredients until well combined. Set aside about 2 tbsp of marinade to reserve for brushing the chicken during grilling.
Pound the chicken to ensure it is even thickness throughout. To do this, I just stick the chicken on a small baking sheet in a single layer sandwiched between 2 layers of paper towels and use my fist to make quick work of this task.
Add chicken breast to a resealable bag, then pour remaining marinade over top and massage around to ensure even distribution. Set aside in refrigerator to marinate for 2-6 hours.
Mix together all salsa ingredients and set aside, covered, in the refrigerator.
Pre-heat grill to 400°. Spray grill grates with non-stick grill spray or brush grates with vegetable oil. Allow excess marinade to drip from the chicken breast before adding to grill. Grill one side for 6 minutes, then flip, brush with marinade, and grill for an additional 4-6 minutes depending on the thickness of the chicken breasts. The internal temperature should just barely reach 165° as you remove the chicken from from the grill.
Allow the chicken to rest for 5 minutes before slicing and serving with salsa and rice (optional).