Prepare the tofu: Press the tofu for at least 10 minutes to drain any excess liquid (see page 11 for more details). Turn the block of tofu on its side and slice it through the middle to make 1-inch (2.5-cm)- thick slabs. You will have 2 to 3 slabs depending on the size and thickness of your tofu.
Prepare the sauce: In a small bowl, mix together all the sauce ingredients. Set aside.
Heat a large, nonstick skillet over medium-high heat. Add the oil. Once hot, add the tofu slabs. Pan-fry the tofu until lightly browned on each side, about 4 minutes per side. Remove the tofu from the pan and then let it cool for 5 minutes before slicing each slab into 1-inch (2.5-cm)-thick strips.
In the same pan, over medium heat, add the sauce and stir well until the sugar dissolves. Increase the heat to medium-high and bring the sauce to a boil. Once boiling, lower the heat to medium. Stir the sauce to prevent the sugar from sticking to the pan and burning. Taste the sauce and feel free to add more sugar, if desired. Afterward, add the sliced tofu to the pan, then scoop the sauce to pour over the top of the tofu pieces. Continue to cook and baste the tofu until it has absorbed some of the sauce, 7 to 8 minutes. Increase the heat to high; once the sauce thickens, about 2 minutes, turn off the heat.
Garnish your tofu with sesame seeds and scallions, if desired. Enjoy your Char Siu Tofu as is, with blanched veggies, with rice or as filling for buns to make gua baos.