What is the possessive form of Bess?

There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the Word: Bess'
Add an apostrophe s ('s) after the existing s at the end of the Word: Bess's

Examples:

The cookies are made from Aunt Bess' recipe.

The cookies are made from Aunt Bess's recipe.