Prepare tofu: Squeeze all the water out of the tofu, then tear into bite-sized chunks. Add soy sauce and white pepper, let sit for 10 minutes. Coat everything well in cornstarch.
Fry tofu: Prepare a pot with at least 2-inches of cooking oil, and heat over medium-high heat. Once the oil is hot (a wooden chopstick inserted should start to bubble), fry the coated tofu in batches for 1-2 minutes, until crispy and light brown. Transfer to a wire rack to drain.
Cook sauce: In a large pan over medium heat, cook garlic and ginger in oil until fragrant. Deglaze the pan with water, then add marmite, sugar, soy sauce, MSG and cornstarch slurry. Reduce the sauce and let thicken, then turn off the heat and add the fried tofu. Make sure each piece is coated in the sauce.
Serve: Garnish with scallions and toasted white sesame seeds. Serve immediately with rice and other dishes.
Notes
you can adjust the soy sauce, marmite and sugar ratios to your liking but keep in mind that marmite has a very strong distinct flavour and needs sugar to balance it out.
you can also use regular firm tofu, but thawed tofu has better texture.
if you'd like to use lesser oil, you can also air-fry/bake the tofu.
Air-fry: spray with oil and cook for 10 minutes at 180C in a preheated air-fryerBake: spray with oil and bake for 15 minutes at 180C in a preheated oven