Mix ¼ cup baking soda with enough water to make a thick paste. The paste should be the same consistency as toothpaste.
Work the paste into the porcelain burn mark with an old toothbrush. It might take a while, but the baking soda will grind away the top few layers of the porcelain to remove the mark. Work until the mark is gone or until it is as light as possible.
Wipe the baking soda paste off with a clean cloth.
Soak the area with water.
Scrub the water into the porcelain with a piece of superfine steel wool to remove the rest of the burn mark. The water will act as a lubricant, which will prevent the steel wool from deeply scratching the porcelain.