13. Make Sauce
While the dough is proofing, make the sauce. Combine the reserved roasting juices with 1/2 cup water, soy sauce, hoisin sauce, and sugar in a small saucepan over medium heat.
While the sauce comes to a simmer, combine the cornstarch with the remaining 2 tablespoons water to make a slurry.
When the sauce comes to a simmer, stir in the slurry. Continue stirring over medium heat until the the sauce thickens and coats the back of a spoon, about 30 seconds. Remove from the heat and set aside.