Add all the marinade ingredients to a bowl and mix until well combined. Marinate the tofu for at least 15 minutes, preferably overnight.
Drain the tofu and transfer ⅓ of the tofu cubes to a freezer bag with 2 tablespoon of cornstarch and toss to coat. Repeat this step with all the remaining tofu cubes and cornstarch. Add more cornstarch if needed. You can also mix the tofu cubes and the cornstarch in a bowl or a shallow dish, but the freezer bag is really convenient and also works amazing.
Cook the tofu cubes in a skillet with some oil over medium-high heat until all sides are golden brown and set aside. If you don't want to use oil, bake the tofu at 400ºF or 200ºC for 15 minutes, then flip each of the tofu cubes (that way they can cook evenly on the other side) and bake for another 15 minutes or until golden brown.
Mix all the sauce ingredients in a bowl until well combined and set aside.
Heat some oil in a skillet and saute the red bell pepper and green bell pepper over medium-high heat for about 5 minutes, stirring occasionally. If you don't want to use oil, just use some water or vegetable stock instead.
Add the chillies (chop them if you want your Kung Pao Tofu spicy and also feel free to add more), garlic and ginger and cook for 1-2 more minutes, stirring frequently.
Add the sauce and cook until it thickens, stirring frequently.
Add the tofu, stir and cook for 1-2 more minutes.
Serve immediately with your favorite toppings (I added chopped peanuts and chopped green onions) or just by itself. Keep leftovers in an airtight container in the fridge for 4 to 5 days.
Notes
You can buy the Hoisin sauce at any Asian supermarket, or make your own. You could also use some extra soy sauce/tamari and vinegar instead. Miso is also a good choice. As Hoisin sauce is sweet, feel free to add more maple syrup or any other sweetener.
The amount of sodium in the recipe is not accurate, as we discard part of the marinade. If you want to reduce the amount of sodium, use low-sodium soy sauce/tamari, or add less and use low-sodium vegetable stock instead of water to get a tasty sauce.
Nutritional info has been calculated by using 2 tablespoon of extra virgin olive oil and it doesn't include the toppings (peanuts and green onions).