Date: 17 Teves 5563
From: Rabbi Moshe Sofer
To: Unknown recipient
The correspondent questions the Ashkenazi custom of including 'bein yom hashvi'i' in Atah Chonantanu and the Rambam's allowance of combining Birkas HaMazon and Havdalah on one cup. The Chasam Sofer defends the custom and explains that when both blessings thematically conclude Shabbos they constitute one matter permitting a shared cup.
Source: Shu"t Chasam Sofer OC §75