I saw a psak given that it is asur to cut a cake with letters made of icing! in the mishna berura is that not talking about real ink? also in the aruch hashulchan (340:23) it seems that it is no problem when the letters are made of edible icing? Please can you get back to me on this psak?
Although the Mishnah Berurah writes that the prohibition mentioned by the Rema does not apply to shapes that are part of the dough itself, and not to writing with a “water-honey” solution, this leniency would not apply to icing. “Water-honey” solution is presumably a very weak form of ink (as stated in Kaf Hachaim 340:33), and is therefore not considered writing at all, whereas icing is strong and solid. This distinction is ruled by Be’er Moshe (6:72), and by Ketzos Hashulchan.
Although the Aruch Hashulchan leans towards leniency in all cases, he, too, writes that in deference to the ruling of the Rema, cake with writing on should be given to children, and not cut by adults.
As mentioned in the last answer, faint cuts in the icing can be prepared before Shabbos.