Start by getting a small saucepan. Place it on medium heat and add the soy sauce, honey, brown sugar, sesame oil, rice vinegar, garlic, and ginger to the pan. Whisk everything together until well combined. You'll notice the sweetness and umami mixing beautifully as it heats up.
Continue to whisk the mixture until it comes just to a boil. Keep an eye on it, as you want to avoid any overflows. The color will deepen slightly as it heats, turning into a glossy mixture that smells divine.
Once boiling, it's time to lower the heat. Decrease it to a gentle simmer. This allows the flavors to meld together beautifully. You'll notice the sauce becoming a little thicker already at this stage.
In a separate small cup or bowl, stir together the water and cornstarch to create a slurry. This step is crucial as it will help thicken your sauce without any lumps. Make sure to whisk it well until there are no clumps left.
Now, while constantly whisking the sauce on the stove, slowly pour the cornstarch slurry into the mixture. This is where the magic happens! You'll see your sauce begin to transform as it thickens right before your eyes.
Let the Teriyaki Sauce simmer for about 5 minutes or so. During this time, it will reach the perfect thickness. If you find it too thick for your liking, feel free to add a little more water to adjust.
Once thickened to your satisfaction, remove the saucepan from heat. Allow it to cool slightly before using. It will continue to thicken as it cools, so don't worry if it's a little runny right off the stove.