Cut tofu into squares. Use a round spoon to remove a part of the tofu to form a round hole in the centre. Sprinkle some corn flour into the hole.
Combine ingredients (A) in a mixing bowl. Add seasoning (B) and mix well to form a sticky paste. Shape into a ball and place in the round hole in the tofu. Arrange the tofu on a plate together with the mushrooms.
Bring water to a boil in the wok, lower the heat and steam tofu for three to four minutes, then take out the plate of tofu. Pour away any excess water.
Bring sauce ingredients to a quick boil in a saucepan. Add thickening and pour sauce over steamed tofu. Garnish before serving.