Cut your block of tofu into ½ inch thick slabs, then cut the slabs into 1- to 2-inch squares. Cut each square on a diagonal to create two triangles.
To make the sauce, first stir together the cold water and cornstarch until there are no lumps. Stir in the soy sauce, vegan oyster sauce, Shaoxing wine, brown sugar, toasted sesame oil, and doubanjiang. Set this aside for now.
Place a large wok over medium heat, then coat the bottom with one tablespoon of the oil. Add the tofu in an even layer (cook it in batches if needed). Pan-fry the tofu triangles for 4 to 5 minutes on each side, until golden brown. Transfer them to a plate when they're done.
Add the remaining tablespoon of oil to the wok and raise the heat to medium-high. Give the oil a minute to heat up, then add the broccoli, onion, and bell pepper. Stir-fry the vegetables for about 5 minutes, until they're tender crisp and lightly charred in spots.
Push the vegetables to the side of the wok, then add the garlic and ginger to the space you just created. Sauté the garlic and ginger for about 30 seconds, stirring constantly, until very fragrant.
Stir the garlic and ginger in with the vegetables, then add the water chestnuts and return the tofu to the wok. Give the sauce a quick stir, then carefully pour it over the tofu and vegetables. Stir everything up and cook the mixture for one to two minutes more, until the sauce has thickened and clings to the tofu and vegetables.
Remove the wok from heat, then serve immediately over rice.