Drain and dry the tofu, then cut into bite-sized cubes.
Mix the soy sauce with the oil, tomato ketchup, vinegar and pepper. Place the tofu cubes in the marinade and marinate with the lid closed for about 30 minutes, turning frequently to the other side.
Heat the butter in a frying pan and roast the tofu cubes in it over medium heat for 5 min. until brown all over.
Serve the tofu cubes on the spot.