New York Consolidated Laws, Estates, Powers and Trusts Law - EPT § 13-2.1 Agreements involving a contract to establish a trust, to make a testamentary provision of any kind, and by a personal representative to answer for the debt or default of a decedent, required to be in writing

(a) Every agreement, promise or undertaking is unenforceable unless it or some note or memorandum thereof is in writing and subscribed by the party to be charged therewith, or by his lawful agent, if such agreement, promise or undertaking:

(b) A contract to make a joint will, or not to revoke a joint will, if executed after the effective date of this paragraph can be established only by an express statement in the will that the instrument is a joint will and that the provisions thereof are intended to constitute a contract between the parties.