In a small mixing bowl, combine the cornstarch and water and mix well. Place to the side.
OPTIONAL: In a saucepan over medium heat, sauté in olive oil the garlic, green onion, and ginger until fragrant (1 to 2 minutes). If you decide to skip this step, simply begin with step 3 in the saucepan over medium heat.
Add the pineapple juice, brown sugar, vinegar, ketchup, soy sauce, water, and cayenne pepper powder. Stir often while bringing the sauce to a boil.
Add the cornstarch/water mix and stir to combine. Continue boiling the sauce until you nearly reach your desired thickness, then remove the sauce from the heat.
Allow the sauce to cool to room temperature (it will continue to thicken while cooling), then serve immediately.