Line a 8-inch square pan with parchment.
In the top of a double boiler set over simmering, not boiling, water, heat chocolate, stirring frequently, until melted. Remove from heat; cool slightly.
In a large mixing bowl, using an electric mixer set on medium speed, beat cream cheese and butter until smoth. Add vanilla and salt. Gradually add powdered sweetener, beating until smooth.
Add the melted chocolate to the cream cheese mixture, beating until combined. Spread mixture in prepared pan; smooth top. Chill until firm, about 1 hour.
To remove fudge from pan, lift up edges of parchment paper. Cut fudge into squares; store in refrigerator or frezzer until ready to use.