Combine the Liquids and Spices: In a medium saucepan, combine the light soy sauce, apple cider vinegar, granulated sugar, pineapple juice, ketchup, Dijon mustard, garlic powder, onion powder, ground ginger, and optional cayenne pepper. Give it a good whisk until the sugar begins to dissolve and the mixture takes on a uniform, light reddish-brown colour.

Bring to a Simmer: Place the saucepan over a medium heat. Stir occasionally. As it heats, you’ll start to notice a sweet and tangy aroma filling your kitchen. Bring the mixture to a gentle simmer, where you see small bubbles forming around the edges of the pan, which should take about 3-5 minutes.

Prepare the Cornflour Slurry: While the sauce is heating, in a small bowl, whisk together the cornflour and cold water until you have a smooth, lump-free slurry. It will look like a milky, opaque liquid.

Thicken the Sauce: Once the sauce is simmering, slowly pour the cornflour slurry into the saucepan, whisking continuously. You'll observe the sauce immediately begin to thicken, transforming from a watery liquid to a glossy, more viscous consistency, much like a thin syrup. Continue to whisk for another minute or two until it reaches your desired thickness.

Taste and Adjust: Remove the saucepan from the heat. Carefully taste a small amount of the sauce (it will be hot!). This is your opportunity to adjust the flavour profile. If you prefer it sweeter, add a tiny bit more sugar. For more tang, a splash of apple cider vinegar can work wonders. The sauce should have a rich, complex aroma that is both sweet and tart.

Cool and Store: Allow the sauce to cool completely at room temperature. As it cools, it will thicken slightly further. Once cooled, transfer the Polynesian sauce into an airtight jar or container. It should have a beautiful, deep amber hue and a smooth, pourable texture. Store it in the refrigerator.