In a pot, bring the soy sauce, oyster sauce, hoisin sauce, honey, and 2 tablespoons of water up to a simmer over medium heat. Whisk in the cornstarch slurry and cook, stirring, until the sauce comes to a boil and starts to thicken. Stir in the char siu cubes and set aside.
Leaving the rolls attached, place the tray buns on a baking sheet and cut a hole into the top of each roll, about ⅔ of the way down. Brush with the melted butter on the insides and outsides of the rolls. Scoop the char siu mix into the holes and place the rolls in the oven and bake until golden, about 5-8 minutes. Remove, let sit for a minute or two and enjoy hot.
