Can one cut a cake or ice-cream (on Shabbos) which has words written on top of it in edible icing or sweets? Can you cut through the letters?
If the letters are written in icing or sweets, meaning something that is external to the cake itself, it is prohibited to cut through the letters themselves (Rema, Orach Chaim 340:3). Instead of cutting through the letters, one should cut between the letters. Alternatively, small cuts should be made in the lettering from before Shabbos, and one may then cut along these cuts on Shabbos.
There is no prohibition on eating the cake with the icing, even if a bite will cause the letters to be ‘erased’ (Mishnah Berurah 340:15).