In a medium pot, whisk together 1 cup of the water, soy sauce, brown sugar, maple syrup, fresh ginger, fresh garlic, ground ginger, ground garlic, and apple cider vinegar.
In a small bowl, whisk together the cornstarch and the remaining ¼ cup of water and add it to the pot.
Place pot over high heat and whisk while bringing to a boil, allowing the sauce to boil for 5 minutes while whisking consistently. Remove from heat and pour into a glass container.
Notes
Tweak the sauce to your liking: By making teriyaki sauce at home, you can tailor the taste (and sodium/sugar levels) to your liking, so feel free to do so and/or experiment with the optional add-ins listed above.
Be patient: Simmer the sauce until it coats the back of the spoon.
To adjust the consistency: For a thin marinade, you could skip the cornstarch entirely. For a slightly thicker teriyaki sauce or glaze, increase the amount of cornstarch used and the simmer time. Just note it will thicken more as it cools.
For more developed flavor: Simmer the sauce for longer to allow the flavors to meld. Just be aware that it will reduce and thicken more, too.