Whisk or stir all of the sauce ingredients together in a small bowl or container. Ensure the cornstarch is completely dissolved. If you find it to be stubborn, try chilling the mixture while you prepare your other ingredients, then stir again just before using.
Place the cornstarch into a small bowl, then add the tofu cubes. Gently stir to completely coat each piece in cornstarch, adding more cornstarch to the bowl if needed.
Place the oil into a large wok, then place the wok over medium heat. Give the oil a minute to heat up. Once it begins to shimmer, add the tofu pieces in an even layer. You can cook the tofu in batches if it won't all fit in the pan at once. Cook the pieces for 4 to 5 minutes on each side, until they're crispy and golden brown. Transfer them to a plate when they're done.
If there's a lot of oil left in the wok, carefully blot some up, until you just have a light coating. If the wok seems very dry, add a dash more oil. Raise the heat to medium-high. Allow the wok to heat up for a minute, then add the zucchini. Stir-fry the zucchini for about 3 minutes, until the pieces are tender-crisp and browned in spots.
Push the zucchini to the side. Add the minced garlic to the space that you just created. Sauté the garlic for about 30 seconds, stirring constantly, until it becomes very fragrant.
Return the tofu to the wok, then carefully pour the sauce over the tofu and zucchini. Continue stir-frying everything for about 1 minute, until the sauce thickens and coats the tofu and zucchini.
Remove the wok from heat, then stir in the scallions and sesame seeds.
Serve the stir-fry immediately, with rice.