Put the slab of tofu in a glass bowl and cover it with water that has just boiled. Leave to stand for 10 minutes, then drain in a colander. Put a plate on top of the tofu, and put something fairly heavy –
Cut the slab of tofu into quarters, and each quarter into quarters again. Brown the tofu pieces in butter, turning them over several times. You may need to do this in 2 or 3 batches.
Mix the ingredients for the marinade in a bowl, and marinade the tofu for at least 10 minutes, making sure that all sides of every piece are coated with the mixture. Then proceed with the donburi.