I have read from several people that cooling it too fast will cause the chocolate to separate from the toffee. One person commented she had put hers in the freezer to cool and the chocolate popped right off. Another person added they thought it was because the chocolate cooled too fast that caused it to separate. These comments were on a recipe site where I was reading the Saltine Toffee recipe, but the problem was the same with the chocolate layer separating from the toffee layer.