Mix the cornstarch with the 6 tablespoon of water in a small bowl and set aside.
Meanwhile, add the soy sauce, ½ cup of water, brown sugar, garlic powder, ground ginger, and pineapple juice in a medium saucepan. Heat over medium heat until it just starts to bubble and the sugar dissolves, about 2 minutes.
Add the cornstarch slurry to the pan and stir to combine. Reduce heat to a low simmer. Simmer, stirring often to prevent burning on the bottom, until the sauce is thickened and coats a spoon, about 10 minutes.
Remove from the heat and let sit for about 5 minutes to cool and thicken a bit more.