Drain and press your extra-firm tofu to remove excess water. Cut it into 1-inch cubes.
Toss the tofu with soy sauce, cornstarch, and black pepper until it's evenly coated.
Add the vegetable oil to a large skillet or wok over medium-high heat.
Once the oil is hot, carefully add the tofu cubes. Cook, stirring occasionally, until they're golden brown and crispy on all sides. Remove the tofu and set aside.
Add the sesame oil to the pan. Toss in the minced garlic and ginger, cooking for just 30 seconds until fragrant.
Stir in the soy sauce, mirin, gochujang paste (start with 1 tablespoon, adding more later if you like heat), ketchup, and brown sugar.
Let the sauce bubble gently for about a minute or two, stirring constantly. It should thicken slightly.
Add the crispy tofu back into the pan with the sauce and toss to coat evenly.
Sprinkle with sesame seeds and sliced green onions and serve over your favorite rice.
