The actress is best known for films like "Seven Brides For Seven Brothers" and "Royal Wedding."
It was given to Moya Smith, a royal staffer, who preserved it with cling film and dated it July 29, 1981.
The slice is in pretty great condition, considering it was made 40 years ago.
The Duchess of York has previously spoken about how hurt she was over not being included at the Duke and Duchess of Cambridge's 2011 nuptials.
From the Duchess of Cambridge doing her own wedding day makeup to the Duke of Cambridge's sweet words at the altar.
The Duke and Duchess of Cambridge appear to recreate their engagement photo in one of the new wedding anniversary pictures.
Assembling the 3-foot cake required removing a door at Buckingham Palace — which Fiona Cairns says Her Majesty certainly noticed.
The Duchess of Sussex told Oprah that she and Harry "got married" three days before their royal wedding.
Clarification comes after a British tabloid obtains a copy of the actual marriage certificate for the Duke and Duchess of Sussex.
A source said efforts to improve diversity were underway before the bombshell interview, but weren't progressing as quickly as hoped.