Add the soy sauce, brown sugar, garlic, ginger, sesame oil, coconut milk, and pineapple juice into an airtight container or durable Ziploc bag. Be sure the ingredients are well combined before adding in the chicken. Place the chicken in the sauce and let marinate in the refrigerator for at least 4 hours, but no more than 24 hours. Any longer and the acidity of the pineapple juice will begin to break down the protein DNA strands in the chicken and cause the texture to be weird.
When you are ready to grill the chicken, take out and let sit on the counter to slightly warm for no more than 30 minutes. While the chicken is resting on the counter, heat the grill to medium-high heat. Grill the chicken until the internal temperature is at least 165°F. Once the chicken reaches that temperature, take off grill, place on a plate and cover with tinfoil.
Place the pineapple on the grill to cook next. This will be very quick so be sure to watch it. Each side only takes 2-3 minutes.
For the Marinade: If you'd like, you can turn the marinade into a sauce to drizzle over the top of the cooked chicken. To do this, pour the marinade into a saucepot, and cook on medium-high until a rolling boil occurs; whisking the entire time so the bottom does not scorch. Once a rolling boil occurs, take 2 Tbsp of corn starch and add it to 1/2 cup of cold water. Mix together until all of the chinks of corn starch are gone. Pour into the cooking marinade and whisk vigorously to combine. The sauce will begin to thicken. let boil, while whisking, for another 3-4 minutes and then remove from heat.
When all is prepared, place the pineapple and chicken on a serving plate and drizzle the prepared sauce over the top. Serve with rice or rice noodles. Or even better, coconut lime rice...which will be another recipe to come.