Put all the ingredients in a blender and purée until smooth to make the seasoning paste.
Rub the seasoning paste under the skin of the spatchcocked chicken and refrigerate for at least 8 hours, or up to two days.
Prepare the basting liquid by simmering vinegar, water, shallots, garlic, fish sauce, soy sauce, black pepper, bay leaves, and sugar in a saucepan. Add butter gradually and strain the mixture.
Bake the chicken by basting it with the prepared liquid, following specific temperature and time instructions.
Let the chicken rest before carving into pieces and serving.
