Combine the sauce ingredients: Add the soy sauce, brown sugar, pineapple juice, minced garlic, grated ginger, and rice vinegar to a small saucepan.
Warm the mixture: Place the saucepan over medium heat and whisk until the brown sugar has dissolved completely.
Simmer gently: Reduce the heat slightly and let the sauce simmer for 8–10 minutes, stirring occasionally.
Make the slurry: In a small bowl, whisk the cornstarch with cold water until completely smooth.
Thicken the sauce: Slowly pour the cornstarch mixture into the simmering sauce while whisking continuously.
Cook until glossy: Continue cooking for about 30–60 seconds, until the sauce becomes smooth, glossy, and thick enough to coat the back of a spoon.
Finish the sauce: Remove the saucepan from the heat and stir in the toasted sesame oil.
Rest before serving: Let the sauce sit for about 5 minutes. It will thicken slightly more as it cools.