Combine the mayonnaise, avocado oil, Dijon mustard, soy sauce, thyme, kosher salt, black pepper, and cayenne pepper in a medium bowl.
Place the chicken breasts in a large zip-top bag or a shallow dish.
Pour the marinade over the chicken, turning to coat evenly. Cover and refrigerate for at least four hours or up to overnight.
Remove the chicken from the refrigerator and discard the remaining marinade.
Preheat your Blackstone griddle to medium-low, aiming for a surface temperature around 350°F (175°C).
Apply a light layer of avocado oil to the griddle surface.
Place the chicken on the griddle and cook for 7 minutes, then flip using tongs.
Cook for another 7 to 8 minutes until a meat thermometer inserted into the thickest part reads 165°F (74°C).
Transfer the chicken to a clean plate, tent loosely with aluminum foil, and let it rest for 5 to 10 minutes before slicing.