Make the Slurry: In a small bowl, whisk together the cornstarch and 1/4 cup water until smooth. Set aside.
Combine the Sauce Base: In a medium saucepan over medium-high heat, combine the pineapple juice, 6 ounces water, brown sugar, granulated sugar, apple cider vinegar, ketchup, and soy sauce. Whisk until the sugars dissolve.
Bring to a Boil: Bring the mixture to a boil and let it boil for 1 minute.
Thicken the Sauce: Whisk in the cornstarch slurry. Continue whisking and cook for 3 to 5 minutes, or until the sauce is thickened and glossy.
Cool and Store: Remove from heat and let cool slightly. Use immediately or transfer to a jar and refrigerate until ready to use.
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Notes
Make-Ahead Tips: This sauce stores beautifully! Make it up to 5 days in advance and refrigerate in a sealed jar or container.Reheating Tips: Reheat gently in a saucepan over low heat, stirring frequently. Add a splash of water or pineapple juice if it becomes too thick.Freezing Tips: Freeze the cooled sauce in a freezer-safe container for up to 2 months. Thaw overnight in the fridge and reheat before using.Sweetness Control: If you prefer your sauce more tangy than sweet, reduce the brown sugar by a couple tablespoons to taste.Vinegar Tip: Apple cider vinegar adds great flavor, but white vinegar can be used in a pinch.