“Inscribing two letters” and “erasing in order to write two letters” are counted among the 39 categories of “melakha” (creative labor) that are forbidden by Torah law on Shabbat. According to the biblical law, these prohibitions apply only where the writing in question is permanent (for example, using ink) and inscribed on a surface (paper) that is permanent (not, for example, on a leaf).
Rabbinical law extends the prohibition to include writing of any kind; hence, one must not create lettering on a cake, for example, using icing.
It is permissible to place existing letters together to form a word (so long as they are not fixed into a frame or stuck together to form a surface). Hence, there is no problem with writing words using chocolate letters, “alphabits” cereal, etc.
Erasing: Concerning cutting a cake that has words inscribed on it, the Rema rules (340, 3): “It is forbidden to cut a cake that has letters inscribed on it, even if one’s sole intention is to eat it, since this represents “erasing”.” Cutting the cake leads one to “erase” the letters. The prohibition applies also to decorative letters made from some other substance that adorn the cake. But if the writing is integral to the food itself (i.e., it was baked in a pan that is shaped like letters) then one may cut it.
The halakhic authority known as the “Dagul Me-Revava” questions the above ruling of the Rema, since cutting the cake involves a.) a rabbinical transgression (writing that is not permanent, on a surface that is not permanent), b. destroying (i.e., destruction of the writing that is not undertaken for the purpose of writing something else instead), and c.) an action undertaken “ki-le-ahar yad” (i.e., in a manner different from the way in which we usually erase letters). He concludes that someone who chooses to be strict in this regard should be strict only with himself, and not insist that other people also observe his custom.
The Mishna Berura rules that we may rely on the opinion of the Dagul Me-Revava with regard to biting or chewing a piece of cake that has lettering on it, but that when cutting a cake with a knife one should be careful not to break letters.
1. One should not inscribe letters or picture on a cake on Shabbat.
2. It is permissible to place existing letters next to each other to form words.
3. Care should be taken not to cut letters with a knife; one need not avoid eating a food that has lettering on it.