Combine the soy sauce, scallion, ginger and bay leaf in a small sauce pan. Cook over medium heat until simmering. Lower the heat and simmer for 5 minutes. Remove the ginger and scallion pieces using a slotted spoon and discard them. Add the brown sugar and five spice powder into the sauce. Stir and cook until dissolved. Combine the cornstarch with 1 tablespoon cold water and stir to dissolve completely, then pour it into the soy sauce mixture. Stir and cook until the sauce thickens enough to coat the back of a spoon. Transfer to a small bowl and set aside.