Make the homemade teriyaki marinade by combining all the ingredients in a small saucepan. (1/2 cup cold water, 1/3 cup brown sugar, 1/4 cup soy sauce, 2 tbsp vinegar, 1 tbsp sesame oil, minced garlic, ginger root, garlic chili paste, and 1 tbsp corn starch) and simmer over medium heat until all of the sugar dissolves.
Prep and cook the tofu
Drain and press your extra-firm tofu to remove as much excess water as possible.
Cut the tofu into 1/2 inch cubes.
Marinate the tofu in the teriyaki marinade for at least 15 minutes. (The teriyaki marinade is the same as the homemade sauce, but without the starch to thicken it. This way, your tofu will be able to soak up the sauce.)
Pour the marinade back into a saucepan. Mix 1 tablespoon of cornstarch with 2 tablespoons of water, then stir the mixture into the marinade. Turn the heat to high and bring to a boil, stirring frequently to thicken the sauce.
Stir-fry the tofu in 1 tbsp of sesame oil over medium-high heat, flipping frequently until the tofu cubes are crispy on all sides.
Prep and cook the vegetables
While the tofu is cooking, wash and cut about 3 cups of any veggies that you want to add to the dish.
Remove the cooked tofu from the skillet, and add an additional tablespoon of sesame oil.
Stir-fry the veggies for about 3-5 minutes over medium-high heat until cooked but not mushy.
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Notes
Make sure to drain and press your tofu well.
Add the teriyaki sauce after cooking to prevent it from sticking and burning.
Marinate the tofu for at least 15 minutes. You can marinate it a day or two in advance, but be sure to keep it refrigerated until ready to cook.