Add the coconut milk to a medium bowl or large measuring cup.
Whisk in the curry powder.
Add the grated fresh ginger.
Add the ground turmeric.
Add the kosher salt.
Add the lime zest.
Add the neutral oil.
Add the black pepper.
Add the lime juice, if using.
Add the brown sugar, if using.
Add the cayenne, if using.
Whisk the marinade until smooth.
Taste the marinade before it touches raw chicken.
The marinade should taste bold, salty, and slightly stronger than sauce.
Add the chicken thighs to a zip-top bag or shallow dish.
Pour the marinade over the chicken.
Turn each piece until coated.
Seal the bag or cover the dish.
Refrigerate for at least 2 hours.
Marinate for 2 to 8 hours.
Remove the chicken from the refrigerator 20 minutes before cooking.
Scrape off heavy excess marinade before cooking.
Cook the chicken until the center reaches 165°F.
For grilling, cook over medium heat for 5 to 7 minutes per side.
For skillet cooking, cook in a lightly oiled pan for 6 to 8 minutes per side.
For oven cooking, bake at 425°F for 22 to 28 minutes.
For air fryer cooking, cook at 380°F for 14 to 18 minutes.
For extra color, broil for 1 to 2 minutes.
Rest the chicken for 5 minutes before slicing.
Do not reuse raw marinade as sauce.
If you want sauce, reserve some marinade before adding chicken.
If using leftover marinade, simmer it hard for several minutes before serving.