Cut the tofu into large bite-size pieces, and chop green onion finely.
Place the tofu on two layers paper towels and drain. After about 15 minutes, the paper towel will be completely wet, so turn it over on a new paper towel and let it sit for another 15 minutes.
Mix the sauce ingredients (sugar, soy sauce, mirin, sake, doubanjiang, mayonnaise) in a small bowl. It's okay if the mayonnaise doesn't mix well.
Heat a frying pan and add oil. Wipe off any excess moisture on the tofu with paper towel. Coat the tofu with cornstarch and place in the frying pan.
Fry all sides well over medium high heat until golden brown. The surface should be crispy. Remove the tofu from the frying pan and transfer to a plate.
Put the sauce ingredients in a frying pan and bring to a boil, and simmer until slightly thickened. Then put the tofu back in and mix it with the sauce. Cook for 2-3 minutes, turning the tofu occasionally. (The mayonnaise should melt during this process.)
Place the tofu on a plate and sprinkle it with the green onions.