In a large pot (which can accommodate luau leaves and octopus) heat 2 tablespoons of the grapeseed oil over medium-high heat and add butter. When it begins to shimmer, add the onion and saute until translucent. Add the luau leaves and pour in chicken stock. The luau leaves will be simmered over low heat for a very long time until tender, about 6 hours. Replenish with water, as needed, during the course of cooking.
About 30 minutes before mealtime, brush the remaining 2 tablespoons of grapeseed oil on the octopus and add to the pot of luau leaves with enough water to cover. Combine the coconut milk and sugar in a bowl. Create a slurry in a separate small bowl by whisking 2 tablespoons water gradually into the cornstarch. Whisk the slurry into the coconut milk mixture, and add to the pot of luau leaves and octopus, and season with salt and pepper. Simmer until the octopus is cooked through, about 20 minutes. Remove octopus to a utility plate to drain and set aside until cool enough to handle.
Remove the octopus head and legs, split in half, then slice and dice small. Return to pot of luau leaves and adjust seasoning, to taste, with salt and pepper as needed.