In a large bowl, whisk together soy sauce, honey, olive oil, 1 tablespoon lime zest, 1/4 cup fresh lime juice, minced garlic, ground cumin, ground coriander, 2 teaspoons salt, yellow curry powder, and cayenne (if using). Stir in 1/2 cup of the coconut milk until smooth.
Remove 3/4 cup of the prepared marinade and transfer it to a container; refrigerate for use as the finishing sauce.
Pat the chicken dry. If breasts are very thick, halve horizontally or lightly pound to even thickness so they cook evenly.
Place the chicken in a zip-top bag, pour the remaining marinade over it, remove excess air, seal, and massage to coat. Marinate in the refrigerator for 2 to 8 hours.
When ready to cook, preheat the grill to about 400°F (medium-high) and clean the grates. Oil the grates by rubbing with a paper towel dipped in vegetable oil held with tongs.
Grill the chicken 4–6 minutes per side, depending on thickness, until an instant-read thermometer reads 160°F at the thickest part (carryover will reach 165°F). Remove with a metal spatula and tent with foil; rest 5–10 minutes.
Meanwhile, combine 1 cup basmati rice, the remaining coconut milk (about 1 1/2 cups total with the remainder of the can), 1/2 cup water, and 1/2 teaspoon salt in a saucepan. Bring to a boil, reduce to low, cover, and simmer 13–18 minutes until liquid is absorbed. Remove from heat and let stand covered 10–15 minutes, then fluff with a fork.
Bring the reserved 3/4 cup marinade to a boil in a small pot and boil 1–2 minutes until slightly reduced and thickened. Remove from heat.
Spoon the reduced sauce over the rested chicken. Serve the chicken over the coconut basmati rice and top with fresh cilantro, a squeeze of lime, and chopped cashews if desired.