What is the Halacha about cutting on Shabbat into a cake that has writing on it made out of frosting?

The Poskim are divided on this question - the R’ma (Orach Chayim 340,3) forbidding cutting into the writing; the Dagul MeRvava - permitting it. Since the question involves a rabbinical rather than a Torah injunction, it is possible to accept the more lenient opinion, but abstaining is a reasonable “Hidur”.