For the filling: With electric mixer on medium speed, beat butter and sugar together until fluffy, about 2 minutes. Beat in vanilla and salt. Beat in marshmallow fluff until incorporated, about 2 minutes. Refrigerate filling until slightly firm, about 30 minutes. (Bowl can be wrapped and refrigerated for up to 2 days.)
Dollop 1/4 cup filling on center of flat side of half the cakes. Top with the flat side of the remaining cakes and gently press until filling spreads to edge of cake. Serve.
(Whoopie pies can be refrigerated in airtight container for up to 3 days.)